Method and apparatus for reporting channel state information in wireless communication system

ABSTRACT

The present invention discloses a method and an apparatus for reporting channel state information. According to one embodiment of the present invention, receiving a channel state information-reference signal (CSI-RS) based on CSI-RS setting information that is provided by a base station; and reporting to the base station the CSI generated by using the CSI-RS, wherein the CSI includes precoding information selected from a specific codebook, wherein elements of the specific codebook are configured based on a precoding vector W, wherein the precoding vector W is 
     
       
         
           
             
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     W1 is a precoding vector applied to a first domain antenna group having a 2D antenna structure, W2 is a precoding vector applied to a second domain antenna group having the 2D antenna structure, and wherein “a” can be a value representing phase difference between the first domain antenna group and the second domain antenna group.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for reporting channelstatus information.

BACKGROUND ART

Multi-Input Multi-Output (MIMO) technology is to improve efficiency indata transmission and reception by using multiple transmitting antennasand multiple receiving antennas instead of a single transmitting antennaand a single receiving antenna. If a single antenna is used, a receiverreceives data through a single antenna path. However, if multipleantennas are used, the receiver receives data through various paths.Accordingly, speed and amount in data transmission may be increased, andcoverage may be increased.

In order to increase multiplexing gain of MIMO operation, channel statusinformation (CSI) may be fed back from a MIMO receiver to a MIMOtransmitter. The receiver may determine CSI by performing channelmeasurement through a predetermined reference signal (RS) from thetransmitter.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method forconfiguring reference signal configuration information for supporting a2-dimensional antenna structure normally efficiently, a method fortransmitting a reference signal, and a method for generating andreporting CSI.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

In an aspect of the present invention, a method for reporting channelstate information (CSI) from a user equipment in a wirelesscommunication system, the method comprising: receiving a channel stateinformation-reference signal (CSI-RS) based on CSI-RS configurationinformation provided by a base station; and reporting to the basestation the CSI generated using the CSI-RS, wherein the CSI includesprecoding information selected from a predetermined codebook, elementsof the predetermined codebook are configured based on a precoding vectorW, the precoding vector W is, W1 is a precoding vector applied to afirst domain antenna group having a 2D antenna structure, W2 is aprecoding vector applied to a second domain antenna group having the 2Dantenna structure, and “a” is a value representing phase differencebetween the first domain antenna group and the second domain antennagroup.

In another aspect of the present invention, A user equipment forreporting channel state information (CSI) in a wireless communicationsystem, the user equipment comprising: a receiver; a transmitter; and aprocessor, wherein the processor is configured to control the receiverto receive a channel state information-reference signal (CSI-RS) basedon CSI-RS configuration information that is provided by a base stationand control the transmitter to report to the base station the CSIgenerated using the CSI-RS, the CSI includes precoding informationselected from a predetermined codebook, elements of the predeterminedcodebook are configured based on a precoding vector W, the precodingvector W is, W1 is a precoding vector applied to a first domain antennagroup having a 2D antenna structure, W2 is a precoding vector applied toa second domain antenna group having the 2D antenna structure, and “a”is a value representing phase difference between the first domainantenna group and the second domain antenna group.

In another aspect of the present invention, a method for receivingchannel state information (CSI) from a base station of a wirelesscommunication system, the method comprising: providing a user equipmentwith channel state information-reference signal (CSI-RS) configurationinformation and transmitting a CSI-RS to the user equipment based on theCSI-RS configuration information; and receiving the CSI, which isgenerated by the user equipment using the CSI-RS, from the userequipment, wherein the CSI includes precoding information selected froma predetermined codebook, elements of the predetermined codebook areconfigured based on a precoding vector W, the precoding vector W is, W1is a precoding vector applied to a first domain antenna group having a2D antenna structure, W2 is a precoding vector applied to a seconddomain antenna group having the 2D antenna structure, and “a” is a valuerepresenting phase difference between the first domain antenna group andthe second domain antenna group.

In another aspect of the present invention, a base station for receivingchannel state information (CSI) in a wireless communication system, thebase station comprising: a receiver; a transmitter; and a processor,wherein the processor is configured to provide a user equipment withchannel state information-reference signal (CSI-RS) configurationinformation and control the transmitter to transmit a CSI-RS to the userequipment based on the CSI-RS configuration information, and control thereceiver to receive the CSI, which is generated by the user equipmentusing the CSI-RS, from the user equipment the CSI includes precodinginformation selected from a predetermined codebook, elements of thepredetermined codebook are configured based on a precoding vector W, theprecoding vector W is, W1 is a precoding vector applied to a firstdomain antenna group having a 2D antenna structure, W2 is a precodingvector applied to a second domain antenna group having the 2D antennastructure, and “a” is a value representing phase difference between thefirst domain antenna group and the second domain antenna group.

The above aspects of the present invention may include the followings.

In another aspect of the present invention,

${{W\; 1} = {\frac{1}{\sqrt{N_{T}}}^{{j2\pi}\; {{nk}/N}}}},{{W\; 2} = {\frac{1}{\sqrt{N_{T}}}^{{j2\pi}\; {{bnk}/N}}}},$

N_(T) is the number of transmitting antennas, n=0, 1, 2, . . . , N−1,k=0, 1, 2, . . . , N_(T)/2−1, and N is the number of beams.

In another aspect of the present invention, “b” is determined based on“a”.

In another aspect of the present invention, b=√{square root over(1−a²)}.

In another aspect of the present invention, the CSI-RS configurationinformation includes an antenna port count parameter and Ntv number ofresource configuration parameters, the antenna port count parameterindicates the number of antennas of the first domain antenna group, andthe Ntv corresponds to the number of antennas of the second domainantenna group.

In another aspect of the present invention, the CSI-RS configurationinformation includes a first antenna port count parameter, a secondantenna port count parameter and a resource configuration parameter, thefirst antenna port count parameter indicates the number of antennas ofthe first domain antenna group, the second antenna port count parameterindicates the number of antennas of the second domain antenna group, andthe resource configuration parameter indicates a resource elementlocation of the CSI-RS for the first domain antenna group correspondingto one antenna of the second domain antenna group.

In another aspect of the present invention, a resource element locationof the CSI-RS for the first domain antenna group corresponding to eachof the other antennas of the second domain antenna group is determinedbased on an offset value corresponding to a value of the second antennaport count parameter.

In another aspect of the present invention, the 2D antenna structure isconfigured by the number of antennas of the second domain antennagroup×the number of antennas of the first domain antenna group.

In another aspect of the present invention, the first domain is ahorizontal domain, and the second domain is a vertical domain.

The aforementioned embodiments and the following detailed description ofthe present invention are only exemplary, and are intended foradditional description of the present invention cited in claims.

Advantageous Effects

According to the present invention, a method for configuring newreference signal configuration information, a method for transmitting areference signal, and a method for generating and reporting CSI may beprovided, whereby a 2-dimensional antenna structure may be supportednormally and efficiently.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating a structure of a radio frame;

FIG. 2 is a diagram illustrating a resource grid at a downlink slot;

FIG. 3 is a diagram illustrating a structure of a downlink subframe;

FIG. 4 is a diagram illustrating a structure of an uplink subframe;

FIG. 5 is a diagram illustrating a wireless communication system havingmultiple antennas;

FIG. 6 is a diagram illustrating an exemplary pattern of CRS and DRS onone resource block pair;

FIG. 7 is a diagram illustrating an example of a DMRS pattern defined inan LTE-A system;

FIG. 8 is a diagram illustrating examples of a CSI-RS pattern defined inan LTE-A system;

FIG. 9 is a diagram illustrating an example of a method for periodicallytransmitting CSI-RS;

FIG. 10 is a diagram illustrating a basic concept of codebook basedprecoding;

FIG. 11 is a diagram illustrating examples constituting 8 transmittingantennas;

FIG. 12 is a diagram illustrating ULA and URA;

FIG. 13 is a diagram illustrating examples of beamforming based on2-dimensional antenna configuration;

FIGS. 14 and 15 are diagrams illustrating a method for allocatingantenna port numbers in a 2-dimensional antenna structure;

FIG. 16 is a diagram illustrating an example of 2-dimensional antennaarray;

FIG. 17 is a flow chart illustrating CSI-RS related operation for a2-dimensional antenna structure according to the present invention; and

FIG. 18 is a diagram illustrating a base station and a user equipmentaccording the preferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The following embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment.

In this specification, the embodiments of the present invention will bedescribed based on data transmission and reception between a basestation and a user equipment. In this case, the base station means aterminal node of a network, which performs direct communication with theuser equipment. A specific operation which has been described as beingperformed by the base station may be performed by an upper node of thebase station as the case may be.

In other words, it will be apparent that various operations performedfor communication with the user equipment in the network which includesa plurality of network nodes along with the base station may beperformed by the base station or network nodes other than the basestation. At this time, the ‘base station’ (BS) may be replaced withterminologies such as a fixed station, Node B, eNode B (eNB), and anaccess point (AP). Also, a relay may be replaced with terminologies suchas a relay node (RN) and a relay station (RS). Also, a ‘terminal’ may bereplaced with terminologies such as a user equipment (UE), a mobilestation (MS), a mobile subscriber station (MSS), and a subscriberstation (SS).

Specific terminologies used in the following description are provided toassist understanding of the present invention, and various modificationsmay be made in the specific terminologies within the range that they donot depart from technical spirits of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention may be supported by standarddocuments disclosed in at least one of wireless access systems, i.e.,IEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE, 3GPP LTE-A(LTE-Advanced) system, and 3GPP2 system. Namely, among the embodimentsof the present invention, apparent steps or parts, which are notdescribed to clarify technical spirits of the present invention, may besupported by the above documents. Also, all terminologies disclosedherein may be described by the above standard documents.

The following technology may be used for various wireless accesstechnologies such as CDMA (code division multiple access), FDMA(frequency division multiple access), TDMA (time division multipleaccess), OFDMA (orthogonal frequency division multiple access), andSC-FDMA (single carrier frequency division multiple access). The CDMAmay be implemented by the radio technology such as UTRA (universalterrestrial radio access) or CDMA2000. The TDMA may be implemented bythe radio technology such as global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by the radio technologysuch as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, andevolved UTRA (E-UTRA). The UTRA is a part of a universal mobiletelecommunications system (UMTS). A 3^(rd) generation partnershipproject long term evolution (3GPP LTE) is a part of an evolved UMTS(E-UMTS) that uses E-UTRA, and adopts OFDMA in a downlink and SC-FDMA inan uplink. LTE-advanced (LTE-A) is an evolved version of the 3GPP LTE.WiMAX may be described by the IEEE 802.16e standard (WirelessMAN-OFDMAReference System) and the advanced IEEE 802.16m standard(WirelessMAN-OFDMA Advanced system). For clarification of thedescription, although the present invention will be described based onthe 3GPP LTE/LTE-A, it is to be understood that technical spirits of thepresent invention are not limited to the 3GPP LTE/LTE-A.

FIG. 1 is a diagram illustrating a structure of a radio frame.

In a cellular OFDM communication system, uplink/downlink data packettransmission is performed in a subframe unit, wherein one subframe isdefined by a given time interval that includes a plurality of OFDMsymbols. The 3GPP LTE standard supports a type 1 radio frame structureapplicable to frequency division duplex (FDD) and a type 2 radio framestructure applicable to time division duplex (TDD).

FIG. 1( a) is a diagram illustrating a structure of a type 1 radioframe. The downlink radio frame includes 10 subframes, each of whichincludes two slots in a time domain. A time required to transmit onesubframe will be referred to as a transmission time interval (TTI). Forexample, one subframe may have a length of 1 ms, and one slot may have alength of 0.5 ms. One slot includes a plurality of OFDM symbols in atime domain and a plurality of resource blocks (RB) in a frequencydomain. Since the 3GPP LTE system uses OFDM in a downlink, OFDM symbolsrepresent one symbol interval. The OFDM symbol may be referred to asSC-FDMA symbol or symbol interval. The resource block (RB) is a resourceallocation unit and may include a plurality of continuous subcarriers inone slot.

The number of OFDM symbols included in one slot may be varied dependingon configuration of a cyclic prefix (CP). Examples of the CP include anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be 7. If the OFDM symbols are configured by the extended CP,since the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is smaller than that of OFDM symbols incase of the normal CP. For example, in case of the extended CP, thenumber of OFDM symbols included in one slot may be 6. If a channelstatus is unstable like the case where the user equipment moves at highspeed, the extended CP may be used to reduce inter-symbol interference.

If the normal CP is used, since one slot includes seven OFDM symbols,one subframe includes 14 OFDM symbols. At this time, first two or threeOFDM symbols of each subframe may be allocated to a physical downlinkcontrol channel (PDCCH), and the other OFDM symbols may be allocated toa physical downlink shared channel (PDSCH).

FIG. 1( b) is a diagram illustrating a structure of a type 2 radioframe. The type 2 radio frame includes two half frames, each of whichincludes five subframes, a downlink pilot time slot (DwPTS), a guardperiod (GP), and an uplink pilot time slot (UpPTS). One subframeincludes two slots. The DwPTS is used for initial cell search,synchronization or channel estimation at the user equipment. The UpPTSis used for channel estimation at the base station and uplinktransmission synchronization of the user equipment. Also, the guardperiod is to remove interference occurring in the uplink due tomultipath delay of downlink signals between the uplink and the downlink.Meanwhile, one subframe includes two slots regardless of the type of theradio frame.

The structure of the radio frame is only exemplary, and variousmodifications may be made in the number of subframes included in theradio frame, the number of slots included in the subframe, or the numberof symbols included in the slot.

FIG. 2 is a diagram illustrating a resource grid at a downlink slot.

One downlink slot includes, but not limited to, seven OFDM symbols in atime domain, and one resource block (RB) includes, but not limited to,twelve subcarriers in a frequency domain. For example, although one slotincludes seven OFDM symbols in case of the normal CP, one slot mayinclude six OFDM symbols in case of the extended CP. Each element on theresource grid will be referred to as a resource element (RE). Oneresource block (RB) includes 12×7(6) resource elements. The numberN^(DL) of resource blocks (RBs) included in the downlink slot depends ona downlink transmission bandwidth. A structure of an uplink slot may bethe same as that of the downlink slot.

FIG. 3 is a diagram illustrating a structure of a downlink subframe.

Maximum three OFDM symbols located at the front of the first slot withinone subframe correspond to a control region to which a control channelis allocated. The other OFDM symbols correspond to a data region towhich a physical downlink shared channel (PDSCH) is allocated.

Examples of the downlink control channel used in the 3GPP LTE systeminclude a PCFICH (Physical Control Format Indicator CHannel), a PDCCH(Physical Downlink Control CHannel), and a PHICH (Physical Hybrid ARQIndicator CHannel). The PCFICH is transmitted from the first OFDM symbolof the subframe, and includes information on the number of OFDM symbolsused for transmission of the control channel within the subframe. ThePHICH is a response to uplink transmission, and includes HARQ ACK/NACK(acknowledgement/negative-acknowledgement) signal. The controlinformation transmitted through the PDCCH will be referred to asdownlink control information (DCI). The DCI includes uplink or downlinkscheduling information or uplink transmission (Tx) power control commandfor a random user equipment group. The PDCCH includes transport formatand resource allocation information of a downlink shared channel(DL-SCH), resource allocation information of an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, resource allocation information of an upperlayer control message such as a random access response transmitted ontothe PDSCH, a set of transmission power control commands of an individualuser equipment within a random user equipment group, transmission powercontrol information, and activation of voice over Internet protocol(VoIP). A plurality of PDCCHs may be transmitted within the controlregion, and the user equipment may monitor the plurality of PDCCHs.

The PDCCH is transmitted by aggregation of one or more continuouscontrol channel elements (CCEs). The CCE is a logic allocation unit usedto provide a PDCCH at a predetermined coding rate based on the status ofa radio channel. The CCE corresponds to a plurality of resource elementgroups (REGs). The format of the PDCCH and the number of available bitsof the PDCCH are determined depending on the correlation between thenumber of CCEs and the coding rate provided by the CCE.

The base station determines a PDCCH format depending on the DCItransmitted to the user equipment, and attaches cyclic redundancy check(CRC) to the control information. The CRC is masked with a radio networktemporary identifier (RNTI) depending on owner or usage of the PDCCH.For example, if the PDCCH is for a specific user equipment, the CRC maybe masked with cell-RNTI (C-RNTI) of the corresponding user equipment.If the PDCCH is for a paging message, the CRC may be masked with apaging indicator identifier (P-RNTI). If the PDCCH is for systeminformation (in more detail, system information block (SIB)), the CRCmay be masked with system information identifier and system informationRNTI (SI-RNTI). The CRC may be masked with a random access RNTI(RA-RNTI) to indicate a random access response that is a response totransmission of a random access preamble of the user equipment.

FIG. 4 is a diagram illustrating a structure of an uplink subframe.

The uplink subframe may be divided into a control region and a dataregion on a frequency domain. A physical uplink control channel (PUCCH)which includes uplink control information is allocated to the controlregion. A physical uplink shared channel (PUSCH) which includes userdata is allocated to the data region. In order to maintain singlecarrier features, one user equipment does not transmit the PUCCH and thePUSCH at the same time. The PUCCH for one user equipment is allocated toresource block (RB) pair for the subframe. Resource blocks (RBs)belonging to the RB pair reserve different subcarriers for two slots.The RB pair allocated to the PUCCH is subjected to frequency hopping ata slot boundary.

Modeling of MIMO System

FIG. 5 is a schematic view illustrating a wireless communication systemhaving multiple antennas.

As shown in FIG. 5( a), if the number of transmitting antennas isincreased to N_(T) and the number of receiving antennas is increased toN_(R), channel transmission capacity is increased theoretically inproportion to the number of antennas unlike that a plurality of antennasare used in only a transmitter or a receiver. Accordingly, it ispossible to improve a transmission rate and remarkably improve frequencyefficiency. As channel transmission capacity is increased, atransmission rate may be increased theoretically as much as a valueobtained by multiplying a maximum transmission rate R₀, whichcorresponds to a case where a single antenna is used, by an increaserate R.

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO communication system that uses four transmittingantennas and four receiving antennas, a transmission rate theoreticallyfour times greater than that of a single antenna system may be obtained.After theoretical capacity increase of the MIMO system has been provedin the middle of 1990, various technologies have been actively studiedto substantially improve a data transmission rate. Also, some of thetechnologies have been already reflected in the standard of variouswireless communications such as third generation mobile communicationand next generation wireless LAN.

Upon reviewing the recent trend of studies related to the MIMO system,active studies are ongoing in view of various aspects such as the studyof information theoretical aspect related to MIMO communication capacitycalculation under various channel environments and multiple accessenvironments, the study of radio channel measurement and modeling of aMIMO system, and the study of time space signal processing technologyfor improvement of transmission reliability and transmission rate.

A communication method in a MIMO system will be described in more detailwith reference to mathematical modeling. In the MIMO system, it isassumed that N_(T) transmitting antennas and N_(R) receiving antennasexist.

First of all, a transmitting signal will be described. If there existN_(T) transmitting antennas, the number of maximum transmissioninformation is N_(T). The transmission information may be expressed asfollows.

s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

Different kinds of transmission power may be applied to each of thetransmission information S₁, s₂, . . . , s_(N) _(T) . At this time,supposing that each transmission power is P₁, P₂, . . . , P_(N) _(T) ,transmission information of which transmission power is controlled maybe expressed as follows.

ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)  [Equation 3]

Also, ŝ may be expressed as follows using a diagonal matrix P.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

It is considered that a weight matrix W is applied to the informationvector ŝ of which transmission power is controlled, so as to obtainN_(T) transmitting signals x₁, x₂, . . . , x_(N) _(T) . In this case,the weight matrix W serves to properly distribute the transmissioninformation to each antenna. Such transmitting signals x₁, X₂, . . . ,x_(N) _(T) may be expressed as follows using a vector X.

$\begin{matrix}{X = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {\quad{{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

In this case, W_(ij) means a weight value between the ith transmittingantenna and the jth information. W may be referred to as a precodingmatrix.

If there exist N_(R) receiving antennas, receiving signals y₁, y₂, . . ., y_(N) _(R) of the respective antennas may be expressed by a vector asfollows.

y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

In case of channel modeling in the MIMO communication system, channelsmay be classified depending on indexes of transmitting and receivingantenna indexes. In this case, a channel that passes from the jthtransmitting antenna to the ith receiving antenna will be expressed ash_(ij). It is noted that index of the receiving antenna is prior toindex of the transmitting antenna in index of h_(ij).

Meanwhile, FIG. 5( b) illustrates channels from N_(T) transmittingantennas from the receiving antenna i. Several channels may be groupedinto one and then may be expressed by a vector type or a matrix type. Asshown in FIG. 5( b), the channels from N_(T) transmitting antennas tothe ith receiving antenna may be expressed as follows.

h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

Accordingly, all channels from N_(T) transmitting antennas to N_(R)receiving antennas may be expressed as follows.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

Since additive white Gaussian noise (AWGN) is actually added to thechannels after the above channel matrix H. AWGN n₁, n₂, . . . , n_(N)_(R) added to each of the N_(R) receiving antennas may be expressed asfollows.

n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

The receiving signals obtained using the above equation modeling may beexpressed as follows.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

In the meantime, the number of rows and columns of the channel matrix Hindicating the channel state is determined by the number of transmittingantennas and the number of receiving antennas. The number of rows in thechannel matrix H is the same as the number N_(R) of receiving antennas,and the number of columns is the same as the number N_(T) oftransmitting antennas. In other words, the channel matrix H may beexpressed by N_(R)×N_(T) matrix.

A rank of the matrix is defined by a minimum number of the number ofrows and the number of columns, which are independent from each other.Therefore, the rank of the matrix cannot have a value greater than thenumber of rows or the number of columns. Rank (rank(H)) of the channelmatrix H may be limited as follows.

rank(H)≦min(N _(T) ,N _(R))  [Equation 11]

The rank may also be defined by the number of eigen values not 0 wheneigen value decomposition is performed for the matrix. Similarly, therank may be defined by the number of singular values not 0 when singularvalue decomposition (SVD) is performed for the matrix. Accordingly, inthe channel matrix, the rank may physically mean a maximum number ofcolumns or rows that may transmit different kinds of information from agiven channel.

In this specification, ‘Rank’ for MIMO transmission represents thenumber of paths that may transmit a signal independently at a specifictime and a specific frequency resource, and ‘the number of layers’represents the number of signal streams transmitted through each path.Generally, since the transmitter transmits layers corresponding to thenumber of ranks used for signal transmission, the ranks are the same asthe number of layers unless mentioned otherwise.

Reference Signal (RS)

When a packet is transmitted in the wireless communication system,signal distortion may occur during transmission of the packet becausethe packet is transmitted through a radio channel. In order to normallyreceive the distorted signal, a receiver should correct distortion ofthe received signal by using channel information. In order to discoverthe channel information, it is required to transmit the signal known byboth a transmitter and the receiver and discover the channel informationusing a distortion level of the signal when the signal is transmittedthrough the channel. In this case, the signal known by both thetransmitter and the receiver will be referred to as a pilot signal or areference signal.

In case that the transmitter or the receiver of the wirelesscommunication system transmits and receives data by using multipleantennas, a channel status between each transmitter and each receivershould be known to receive a normal signal. Accordingly, a separatereference signal should be provided per transmitting antenna.

In the wireless communication system, the reference signal (RS) may bedivided into two types in accordance with its purpose. Examples of thereference signal include a reference signal used for acquisition ofchannel information and a reference signal used for data demodulation.Since the former reference signal is intended for acquisition of channelinformation on the downlink through the user equipment, it needs to betransmitted through a wideband. Also, the former reference signal shouldbe received and measured even by a user equipment that does not receivedownlink data for a specific subframe. This reference signal foracquisition of channel information may be used even for measurement ofhandover. The latter reference signal is transmitted from the basestation together with a corresponding resource when the base stationtransmits downlink data. In this case, the user equipment may performchannel estimation by receiving the corresponding reference signal,whereby the user equipment may demodulate the data. This referencesignal for data demodulation should be transmitted to a region to whichdata are transmitted.

The existing 3GPP LTE system (for example, 3GPP LTE release-8) definestwo types of downlink RSs for unicast service. The one of the downlinkreference signals is a common reference signal (CRS), and the other oneis a dedicated reference signal (DRS). The CRS is used for bothinformation acquisition of channel status and measurement for handover,and may be referred to as a cell-specific RS. The DRS is used for datademodulation, and may be referred to as a UE-specific RS. In theexisting 3GPP LTE system, the DRS may be used for data demodulationonly, and the CRS may be used for both acquisition of channelinformation and data demodulation.

The CRS is a cell-specific RS and is transmitted to a wideband persubframe. The CRS for maximum four antenna ports may be transmitted inaccordance with the number of transmitting antennas of the base station.For example, if the number of transmitting antennas of the base stationis 2, CRS for antenna ports 0 and 1 may be transmitted. If the number oftransmitting antennas of the base station is 4, CRS for antenna ports 0to 3 may be transmitted respectively.

FIG. 6 is a diagram illustrating an exemplary pattern of CRS and DRS onone resource block pair.

In the example of the reference signal pattern in FIG. 6, patterns ofCRS and DRS are provided on one resource block pair (in case of normalCP, 14 OFDM symbols on the time×12-subcarriers on the frequency) in thesystem that the base station supports four transmitting antennas. InFIG. 6, resource elements remarked with ‘R0’, ‘R1’, ‘R2’ and ‘R3’represent positions of the CRS for antenna port indexes 0, 1, 2 and 3.Meanwhile, in FIG. 6, a resource element marked with ‘D’ represents theposition of the DRS defined in the LTE system.

The LTE-A system which is an evolved version of the LTE system maysupport maximum eight transmitting antennas on the downlink.Accordingly, reference signals for maximum eight transmitting antennasshould also be supported. In the LTE system, since downlink referencesignals are defined for maximum four antenna ports only, if the basestation includes minimum four downlink transmitting antennas to maximumeight downlink transmitting antennas in the LTE-A system, referencesignals for these antenna ports should additionally be defined. Thereference signals for maximum eight transmitting antenna ports may beconsidered for two types of reference signals, i.e., the referencesignal for channel measurement and the reference signal for datademodulation as described above.

One of important considerations in designing the LTE-A system isbackward compatibility. Backward compatibility means that the LTE userequipment of the related art should be operated normally even in theLTE-A system. In view of reference signal transmission, if referencesignals for maximum eight transmitting antenna ports should be definedadditionally in the time-frequency domain to which CRS defined in theLTE standard is transmitted to a full band every subframe, RS overheadbecomes too great. Accordingly, it should be considered that RS overheadis reduced in newly designing RS for maximum eight antenna ports.

The reference signal newly introduced in the LTE-A system may be dividedinto two types. One of the reference signals is a channel statusinformation-reference signal (CSI-RS) which is the RS for channelmeasurement for selecting transmission rank, modulation and codingscheme (MCS), and precoding matrix index (PMI), and the other one is ademodulation RS (DMRS) which is the RS for demodulation of datatransmitted through maximum eight transmitting antennas.

The CSI-RS for channel measurement is designed for channel measurementmainly unlike the existing CRS used for channel measurement, handovermeasurement, and data demodulation. The CSI-RS may also be used forhandover measurement. Since the CSI-RS is transmitted only to obtainchannel status information, it may not be transmitted per subframeunlike the CRS of the existing LTE system. Accordingly, in order toreduce overhead of the CSI-RS, the CSI-RS may be designed to beintermittently (for example, periodically) be transmitted on the timeaxis.

If data are transmitted on a random downlink subframe, a dedicated DMRSis transmitted to the user equipment for which data transmission isscheduled. In other words, the DMRS may be referred to as a UE-specificRS. The DMRS dedicated for a specific user equipment may be designed tobe transmitted from only the resource region for which the correspondinguser equipment is scheduled, that is, the time-frequency domain to whichdata for the corresponding user equipment are transmitted.

FIG. 7 is a diagram illustrating an example of DMRS pattern defined inthe LTE-A system.

FIG. 7 illustrates a position of a resource element where DMRSs aretransmitted on one resource block pair (in case of normal CP, 14 OFDMsymbols on the time×12 subcarriers on the frequency) to which downlinkdata are transmitted. The DMRS may be transmitted four antenna ports(antenna port indexes 7, 8, 9 and 10) defined additionally in the LTE-Asystem. The DMRSs for different kinds of antenna ports may be identifiedfrom one another in such a manner that they are located on differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols) (that is, the DMRSs may be multiplexed in accordance with FDMand/or TDM mode). Also, the DMRSs for different antenna ports located onthe same time-frequency resource may be identified from one another byorthogonal codes (that is, the DMRSs may be multiplexed in accordancewith CDM mode). In the example of FIG. 7, the DMRSs for the antennaports 7 and 8 may be located on the resource elements (REs) of DMRS CDMgroup 1, and may be multiplexed by orthogonal codes. Likewise, in theexample of FIG. 7, the DMRSs for the antenna ports 9 and 10 may belocated on the resource elements (REs) of DMRS CDM group 2, and may bemultiplexed by orthogonal codes.

When the base station transmits the DMRS, the same precoding as thatapplied to data is applied to the DMRS. Accordingly, channel informationestimated by the user equipment using the DMRS (or UE-specific RS) isthe precoded channel information. The user equipment may easily performdata demodulation by using the precoded channel information estimatedthrough the DMRS. However, since the user equipment cannot know theprecoding information applied to the DMRS, the user equipment cannotacquire channel information, which is not precoded, from the DMRS. Theuser equipment may acquire channel information, which is not precoded,by using a separate reference signal in addition to the DMRS, that is,the aforementioned CSI-RS.

FIG. 8 is a diagram illustrating examples of CSI-RS patterns defined inthe LTE-A system.

FIG. 8 illustrates a position of a resource element where CSI-RSs aretransmitted on one resource block pair (in case of normal CP, 14 OFDMsymbols on the time×12 subcarriers on the frequency) to which downlinkdata are transmitted. One of CSI-RS patterns in FIG. 8( a) to FIG. 8( e)may be used for a random downlink subframe. The CSI-RS may betransmitted for eight antenna ports (antenna port indexes 15, 16, 17,18, 19, 20, 21 and 22) defined additionally in the LTE-A system. TheCSI-RSs for different antenna ports may be identified from one anotherin such a manner that they are located on different frequency resources(subcarriers) and/or different time resources (OFDM symbols) (that is,the CSI-RSs may be multiplexed in accordance with FDM and/or TDM mode).Also, the CSI-RSs for different antenna ports located on the sametime-frequency resource may be identified from one another by orthogonalcodes (that is, the CSI-RSs may be multiplexed in accordance with CDMmode). In the example of FIG. 8( a), the CSI-RSs for the antenna ports15 and 16 may be located on the resource elements (REs) of CSI-RS CDMgroup 1, and may be multiplexed by orthogonal codes. In the example ofFIG. 8( a), the CSI-RSs for the antenna ports 17 and 18 may be locatedon the resource elements (REs) of CSI-RS CDM group 2, and may bemultiplexed by orthogonal codes. In the example of FIG. 8( a), theCSI-RSs for the antenna ports 19 and 20 may be located on the resourceelements (REs) of CSI-RS CDM group 3, and may be multiplexed byorthogonal codes. In the example of FIG. 8( a), the CSI-RSs for theantenna ports 21 and 22 may be located on the resource elements (REs) ofCSI-RS CDM group 4, and may be multiplexed by orthogonal codes. The sameprinciple described based on FIG. 8( a) may be applied to FIG. 8( b) toFIG. 8( e).

The RS patterns of FIGS. 6 and 8 are only exemplary, and variousembodiments of the present invention are not limited to a specific RSpattern. In other words, various embodiments of the present inventionmay equally be applied to even a case where RS pattern different fromthose of FIGS. 6 and 8 is defined and used.

CSI-RS Configuration

As described above, in the LTE-A system that supports maximum eighttransmitting antennas on the downlink, the base station should transmitCSI-RSs for all antenna ports. Since transmission of the CSI-RSs formaximum eight transmitting antenna ports every subframe has a drawbackin that overhead is too great, the CSI-RSs should be transmittedintermittently on the time axis without being transmitted everysubframe, to reduce overhead. Accordingly, the CSI-RSs may betransmitted periodically at a period of integer multiple of one subframeor may be transmitted at a specific transmission pattern.

At this time, the transmission period or transmission pattern of theCSI-RS may be configured by the network (for example, base station). Inorder to perform measurement based on the CSI-RS, the user equipmentshould know CSI-RS configuration for each antenna port of a cell (ortransmission point (TP) to which the user equipment belongs. CSI-RSconfiguration may include downlink subframe index for which the CSI-RSis transmitted, time-frequency positions (for example, CSI-RS patternsthe same as those of FIG. 8( a) to FIG. 8( e)) of CSI-RS resourceelements (REs) within a transmission subframe, and CSI-RS sequence (usedfor CSI-RS and generated pseudo-randomly in accordance with apredetermined rule on the basis of slot number, cell ID, CP length,etc.). In other words, a plurality of CSI-RS configurations may be usedby a given base station, and the base station indicate CSI-RSconfiguration, which will be used for user equipment(s) within a cell,among the plurality of CSI-RS configurations.

The plurality of CSI-RS configurations may include or not include oneCSI-RS configuration assumed by the user equipment that transmissionpower of the CSI-RS is not 0 (non-zero). Also, the plurality of CSI-RSconfigurations may include or not include one or more CSI-RSconfigurations assumed by the user equipment that transmission power ofthe CSI-RS is 0.

Also, each bit of parameters (for example, 16-bit bitmap ZeroPowerCSI-RSparameter) for CSI-RS configuration of the transmission power of 0 maycorrespond to CSI-RS configuration (or REs to which the CSI-RS may beallocated in accordance with CSI-RS configuration) in accordance with anupper layer, and the user equipment may assume that the transmissionpower at the CSI-RS REs of CSI-RS configuration corresponding to a bitset to 1 in the corresponding parameter is 0.

Also, since the CSI-RSs for the respective antenna ports are required tobe identified from one another, resources to which the CSI-RSs for therespective antenna ports are transmitted should be orthogonal to oneanother. As described with reference to FIG. 8, the CSI-RSs for therespective antenna ports may be multiplexed in accordance with FDM, TDMand/or CDM mode by using orthogonal frequency resources, orthogonal timeresources and/or orthogonal code resources.

When the base station notifies the user equipment within the cell ofCSI-RS information (CSI-RS configuration), the base station should firstnotify the user equipment of time-frequency information into which theCSI-RSs for the respective antenna ports are mapped. In more detail, thetime information may include subframe numbers to which the CSI-RSs aretransmitted, a transmission period of CSI-RSs, offset of subframe towhich the CSI-RSs are transmitted, and OFDM symbol number to whichCSI-RS resource element (RE) of a specific antenna is transmitted. Thefrequency information may include frequency spacing to which CSI-RSresource element (RE) of a specific antenna is transmitted, offset orshift value of RE on a frequency axis, etc.

FIG. 9 is a diagram illustrating an example of a method for periodicallytransmitting CSI-RS.

The CSI-RS may be transmitted at a period of integer multiple (forexample, 5-subframe period, 10-subframe period, 20-subframe period,40-subframe period, or 80-subframe period) of one subframe.

In FIG. 9, one radio frame includes 10 subframes (subframe numbers 0 to9). For example, in FIG. 9, a transmission period of the CSI-RS of thebase station is 10 ms (that is, 10 subframes), and CSI-RS transmissionoffset is 3. The offset value may be varied depending on respective basestations such that the CSI-RSs of several cells may uniformly bedistributed on the time. If the CSI-RS is transmitted at a period of 10ms, the offset value may have one of 0 to 9. Similarly, if the CSI-RS istransmitted at a period of 5 ms, the offset value may have one of 0 to4, if the CSI-RS is transmitted at a period of 20 ms, the offset valuemay have one of 0 to 19, if the CSI-RS is transmitted at a period of 40ms, the offset value may have one of 0 to 39, and if the CSI-RS istransmitted at a period of 80 ms, the offset value may have one of 0 to79. This offset value represents a value of subframe at which the basestation starts CSI-RS transmission at a predetermined period. If thebase station notifies the user equipment of the transmission period andoffset value of the CSI-RS, the user equipment may receive the CSI-RS ofthe base station at the corresponding subframe position by using thecorresponding value. The user equipment may measure a channel throughthe received CSI-RS and, as a result, report information such as CQI,PMI and/or RI (Rank Indicator) to the base station. In thisspecification, CQI, PMI, and RI may be referred to as CQI (or CSI)except that they are described separately. Also, the information relatedto the CSI-RS may commonly be applied to the user equipments within thecell as cell-specific information. Also, the CSI-RS transmission periodand offset may be designated separately for each CSI-RS configuration.For example, separate CSI-RS transmission period and offset may be setto the CSI-RS configuration indicating CSI-RS transmitted at atransmission power of 0 as described later and CSI-RS configurationindicating CSI-RS transmitted at a transmission power of non-zero.

Unlike the CRS transmitted at all the subframes at which the PDSCH maybe transmitted, the CSI-RS may be set such that the CSI-RS istransmitted at some subframes only. For example, CSI subframe setsC_(CSI,0) and C_(CSI,1) may be configured by the upper layer. CSIreference resource (that is, predetermined resource region which becomesa reference of CSI calculation) may belong to either C_(CSI,0) orC_(CSI,1), or may not belong to both C_(CSI,0) and C_(CSI,1).Accordingly, if the CSI subframe sets C_(CSI,0) and C_(CSI,1) areconfigured by the upper layer, the user equipment may not expect thatCSI reference resource existing at a subframe that does not belong toany one of the CSI subframe sets will be triggered (or indication of CSIcalculation).

Also, the CSI reference resource may be set on a valid downlinksubframe. The valid downlink subframe may be set as a subframe thatsatisfies various requirements. One of the requirements may be thesubframe that belongs to the CSI subframe set linked to periodic CSIreport if the CSI subframe set is configured for the user equipment incase of periodic CSI report.

Also, the user equipment may obtain CQI indexes from the CSI referenceresource by considering the following assumptions (for details, see 3GPPTS 36.213).

-   -   First three OFDM symbols of one subframe are reserved by control        signaling.    -   There is no resource element used by a primary synchronization        signal, a secondary synchronization signal, or physical        broadcast signal (PBCH).    -   CP length of non-MBSFN subframe    -   Redundancy version is 0    -   If the CSI-RS is used for channel measurement, a PDSCH EPRE        (Energy Per Resource Element) to CSI-RS EPRE ratio depends on a        predetermined rule.    -   In case of CSI report in a transmission mode 9 (that is, mode        that supports maximum eight-layer transmission), if PMI/RI        report is configured for the user equipment, it is assumed that        DMRS overhead is matched with rank which is reported most        recently (for example, since DMRS overhead on one resource block        pair is 12 REs in case of two or more antenna ports (that is,        less than rank 2) as described with reference to FIG. 7 but is        24 REs in case of three or more antenna ports (that is, more        than rank 3), CQI index may be calculated by assuming DMRS        overhead corresponding to the rank value which is reported most        recently).    -   RE is not allocated to CSI-RS and 0-power CSI-RS.    -   RE is not allocated to positioning RS (PRS).    -   PDSCH transmission scheme depends on a transmission mode (which        may be default mode) currently set for the user equipment.    -   A PDSCH EPRE to cell-specific reference signal EPRE ratio        depends on a predetermined rule.

This CSI-RS configuration may be notified from the base station to theuser equipment by using RRC (Radio Resource Control) signaling, forexample. In other words, information on the CSI-RS configuration may beprovided to each user equipment within the cell by using dedicated RRCsignaling. For example, the base station may notify the user equipmentof CSI-RS configuration through RRC signaling when the user equipmentestablishes connection with the base station through initial access orhandover. Alternatively, when the base station transmits RRC signalingmessage, which requires channel status feedback based on CSI-RSmeasurement, to the user equipment, the base station may notify the userequipment of the CSI-RS configuration through corresponding RRCsignaling message.

In the meantime, the time position where the CSI-RS exists, that is,cell-specific subframe setup period and cell-specific subframe offsetmay be listed as illustrated in Table 1 below.

TABLE 1 CSI-RS subframe CSI-RS period CSI-RS subframe offsetconfiguration I_(CSI-RS) T_(CSI-RS) (subframe) Δ_(CSI-RS) (subframe) 0-45 I_(CSI-RS)  5-14 10 I_(CSI-RS) - 5 15-34 20 I_(CSI-RS) - 15 35-74 40I_(CSI-RS) - 35  75-154 80 I_(CSI-RS) - 75

As described above, the parameter I_(CSI-RS) may be set separately forthe CSI-RS assumed by the user equipment that the transmission power isnot 0 and the CSI-RS assumed by the user equipment that the transmissionpower is 0. The subframe that includes the CSI-RS may be expressed bythe following Equation 12 (in Equation 12, n_(f) is a system framenumber, and n_(s) is a slot number).

(10n _(f) +└n _(s)/2┘−Δ_(CSI-RS))mod T _(CSI-RS)=0  [Equation 12]

A CSI-RS-Config information element (IE) defined as illustrated in Table2 below may be used to specify CSI-RS configuration.

TABLE 2 CSI-RS-Config information elements -- ASN1STARTCSI-RS-Config-r10 ::= SEQUENCE {   csi-RS-r10   CHOICE {    releaseNULL,    setup SEQUENCE {      antennaPortsCount-r10    ENUMERATED {an1,an2,    an4, an8},      resourceConfig-r10    INTEGER (0..31),     subframeConfig-r10      INTEGER (0..154),      p-C-r10    INTEGER(−8..15)    }   }         OPTIONAL,   -- Need ON   zeroTxPowerCSI-RS-r10CHOICE {    release NULL,    setup SEQUENCE {     zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE      (16)),     zeroTxPowerSubframeConfig-r10  INTEGER (0..154)    }   }        OPTIONAL    -- Need ON } -- ASN1STOP

In Table 2, the antenna port count parameter antennaPortsCountrepresents the number of antenna ports (that is, CSI-RS ports) used forCSI-RS transmission, and an1 corresponds to 1 and an2 corresponds to 2.

In Table 2, the p_C parameter represents a PDSCH EPRE (Energy PerResource Element) to CSI-RS EPRE ratio assumed when the user equipmentUE derives CSI feedback.

In Table 2, the resource configuration parameter resourceConfig has avalue that determines a position of a resource element to which theCSI-RS is mapped on the RB pair as illustrated in FIG. 8.

In Table 2, the subframe configuration parameter subframeConfigcorresponds to I_(CSI-RS) in Table 1.

In Table 2, zero TxPowerResourceConfigList and zeroTxPowerSubframeConfig respectively correspond to resourceConfig andsubframeConfig for the CSI-RS of the transmission power of 0.

Details of the CSI-RS configuration IE in Table 2 will be understoodwith reference to the standard document TS 36.331.

Channel Status Information (CSI)

The MIMO scheme may be divided into an open-loop system and aclosed-loop system. The open-loop MIMO scheme means that a MIMOtransmitter performs MIMO transmission without feedback of channelstatus information from a MIMO receiver. The closed-loop MIMO schememeans that the MIMO transmitter performs MIMO transmission by using thechannel status information fed back from the MIMO receiver. In theclosed-loop MIMO scheme, each of the transmitter and the receiver mayperform beamforming on the basis of the channel status information toobtain multiplexing gain of MIMO transmitting antennas. The transmitter(for example, base station) may allocate an uplink control channel or anuplink shared channel to the receiver (for example, user equipment), sothat the receiver may feed the channel status information back.

The user equipment may perform estimation and/or measurement for adownlink channel by using the CRS and/or the CSI-RS. The channel statusinformation (CSI) which is fed back from the user equipment to the basestation may include a rank indicator (RI), a precoding matrix index(PMI), and a channel quality indicator (CQI).

The RI is information for a channel rank. The rank of the channel meansa maximum number of layers (or streams) that may transmit differentkinds of information through the same time-frequency resources. Sincethe rank value is mainly determined by long term padding of a channel,the rank value may generally be fed back in accordance with a longerperiod (that is, less frequently) than those of the PMI and the CQI.

The PMI is information for a precoding matrix used for transmission fromthe transmitter, and is a value that reflects spatial features of achannel. Precoding means that transmission layers are mapped intotransmitting antennas, and layer-antenna mapping relation may bedetermined by a precoding matrix. The PMI corresponds to a precodingmatrix index of the base station preferred by the user equipment on thebasis of a metric such as a signal-to-interference plus noise ratio(SINR). In order to reduce feedback overhead of precoding information,the transmitter and the receiver previously share a codebook thatincludes various precoding matrixes, and only an index indicating aspecific precoding matrix in the corresponding codebook may be fed back.For example, the PMI may be determined on the basis of RI which isreported most recently.

The CQI is the information indicating channel quality or channelstrength. The CQI may be expressed by MCS combination which ispreviously determined. In other words, the CQI index represents acorresponding modulation scheme and a code rate. The CQI may becalculated by assuming that the PDSCH may be received without exceedinga predetermined error probability (for example, 0.1) on the assumptionthat a specific resource region (for example, region specified by validsubframe and/or physical resource block) is set to CQI referenceresource and PDSCH transmission exists in the corresponding CQIreference resource. Generally, the CQI becomes a value that reflectsreceived SINR that may be obtained if the base station configures aspatial channel by using the PMI. For example, the CQI may be calculatedon the basis of RI and/or PMI which is reported most recently.

In the system (for example, LTE-A system) that supports extended antennaconfiguration, it is considered that additional multi-user diversity isacquired using a multi-user-MIMO (MU-MIMO) scheme. In case of theMU-MIMO scheme, since an interference channel exists between the userequipments multiplexed in an antenna domain, if the base stationperforms downlink transmission by using channel status information fedback from one of multiple user equipments, it is required thatinterference should not occur with the other user equipments.Accordingly, in order that MU-MIMO operation is performed normally,channel status information having exactness higher than that of theSU-MIMO scheme should be fed back.

A new CSI feedback method improved from CSI, which includes the existingRI, PMI and CQI, may be used such that the channel status informationmay be measured and reported more exactly. For example, the precodinginformation fed back from the receiver may be indicated by combinationof two PMIs (for example, i1 and i2). As a result, more exact PMI may befed back, and more exact CQI may be calculated and reported on the basisof the more exact PMI.

In the meantime, the CSI may periodically be transmitted through thePUCCH, or may aperiodically be transmitted through the PUSCH. Also,various report modes may be defined depending on which one of RI, firstPMI (for example, W1), second PMI (for example, W2) and CQI is fed backand whether PMI and/or CQI which is fed back is for wideband (WB) orsubband (SB).

CQI Calculation

Hereinafter, CQI calculation will be described in detail on theassumption that a downlink receiver is a user equipment. However, thedescription in the present invention may equally be applied to a relayas a downlink reception entity.

A method for configuring/defining a resource (hereinafter, referred toas reference resource), which becomes a reference of CQI calculation,when the user equipment reports CSI will be described. First of all,definition of CQI will be described in more detail.

The CQI reported by the user equipment corresponds to a specific indexvalue. The CQI index is a value indicating a modulation scheme, coderate, etc., which correspond to the channel status. For example, the CQIindexes and their definition may be given as illustrated in Table 3.

TABLE 3 CQI index modulation code rate × 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

Based on observation which is not limited by time and frequency, theuser equipment may determine the highest CQI index, which satisfies apredetermined requirement of CQI indexes 1 to 15 of Table 3, for eachCQI value reported at an uplink subframe n. The predeterminedrequirement may be defined that a single PDSCH transmission block, whichreserves a group of downlink physical resource blocks referred to as CQIreference resources, may be received at a transmission block errorprobability that does not exceed 0.1 (that is, 10%), in accordance withcombination of a modulation scheme (for example, MCS) and a transmissionblock size (TBS), which correspond to the corresponding CQI index. Ifthe CQI index 1 does not satisfy the above requirement, the userequipment may determine CQI index 0.

In case of a transmission mode 9 (corresponding to maximum 8-layertransmission) and a feedback report mode, the user equipment may performchannel measurement for calculating a CQI value reported at an uplinksubframe n, on the basis of the CSI-RS only. In case of the othertransmission mode and corresponding report modes, the user equipment mayperform channel measurement for CQI calculation on the basis of the CRS.

If the following requirements are all satisfied, combination of themodulation scheme and the transmission block size may correspond to oneCQI index. The combination may be signaled for transmission on the PDSCHat the CQI reference resource in accordance with a related transmissionblock size table, the modulation scheme is indicated by thecorresponding CQI index, and if combination of the transmission blocksize and the modulation scheme is applied to the reference resource, avalid channel code rate closest to a code rate indicated by thecorresponding CQI index corresponds to the above requirement. If two ormore combinations of the transmission block size and the modulationscheme are close to the code rate indicated by the corresponding CQIindex at the same level, combination having the minimum transmissionblock size may be determined.

The CQI reference resource is defined as follows.

The CQI reference resource in the frequency domain is defined by a groupof downlink physical resource blocks corresponding to a band to whichthe obtained CQI value is related.

The CQI reference resource in the time domain is defined by singledownlink subframe n-nCQI_ref. In this case, in case of periodic CQIreport, nCQI_ref is determined as a value which enables the downlinksubframe n-nCQI_ref to correspond to a valid downlink subframe whilebeing the smallest of values greater than 4. In case of aperiodic CQIreport, nCQI_ref is determined as CQI reference resource which is thesame downlink subframe as a valid downlink subframe corresponding to CQIrequest (or subframe for which CQI request is received) at an uplink DCIformat (that is, PDCCH DCI format for providing uplink schedulingcontrol information to the user equipment). Also, in case of aperiodicCQI report, nCQI_ref is 4 and the downlink subframe n-nCQI_refcorresponds to the valid downlink subframe, wherein the downlinksubframe n-nCQI_ref may be received after the subframe corresponding toCQI request (or subframe for which CQI request is received) at a randomaccess response grant. In this case, the valid downlink subframe meansthe downlink subframe that is set to the downlink subframe for thecorresponding user equipment UE, is not the MBSFN subframe except thetransmission mode 9, does not include a DwPTS field if the length ofDwPTS is less than 7680*Ts (Ts=1/(15000×2048) second), and does notbelong to a measurement gap configured for the corresponding UE. Ifthere is no valid downlink subframe for the CQI reference resource, CQIreport may be omitted for the uplink subframe n.

The CQI reference resource in a layer region is defined as random RI andPMI based on CQI.

In order that the user equipment derives CQI index from the CQIreference resource, the followings may be assumed: (1) first three OFDMsymbols of the downlink subframe are used for control signaling; (2)there is no resource element used by a primary synchronization signal, asecondary synchronization signal or a physical broadcast channel; (3)the CQI reference resource has a CP length of non-MBSFN subframe; (4)redundancy version is 0; (5) if CSI-RS is used for channel measurement,a PDSCH EPRE to CSI-RS EPRE ratio has a predetermined value signaled bythe upper layer; (6) PDSCH transmission scheme (single antenna porttransmission, transmission diversity, spatial multiplexing, MU-MIMO,etc.) defined for each transmission mode is currently set for thecorresponding UE (default mode may be provided); and (7) if CRS is usedfor channel measurement, the PDSCH EPRE to CRS EPRE ratio may bedetermined depending on a predetermined rule. Details related to CQIdefinition may be understood with reference to 3GPP TS36.213.

In short, the downlink receiver (for example, user equipment) may set aprevious specific single subframe to the CQI reference resource based onthe time when CQI calculation is currently performed, and may calculateCQI value from the corresponding CQI reference resource to satisfy thecondition that error probability does not exceed 10% when the PDSCH istransmitted from the base station.

Codebook Based Precoding Scheme

Precoding for properly distributing transmission information inaccordance with a channel status of each antenna may be used to supportmulti-antenna transmission. A codebook based precoding scheme means thata transmitter and a receiver previously defines a set of precodingmatrixes, the receiver feeds the most suitable precoding matrix (thatis, precoding matrix index (PMI)) back to the transmitter by measuringchannel information from the transmitter, and the transmitter appliesproper precoding to signal transmission on the basis of PMI. Since thecodebook based precoding scheme selects a proper precoding matrix of theset of the precoding matrixes, although optimized precoding is alwaysnot used, feedback overhead may be reduced as compared with thatoptimized precoding information is explicitly fed back to actual channelinformation.

FIG. 10 is a diagram illustrating a basic concept of codebook basedprecoding.

According to the codebook based precoding scheme, the transmitter andthe receiver shares codebook information that includes a predeterminednumber of precoding matrixes which are previously determined inaccordance with a transmission rank, the number of antennas, etc. Inother words, if feedback information is finite, the precoding basedcodebook scheme may be used. The receiver may measure the channel statusthrough a received signal and feed information on infinite number ofpreferred precoding matrixes (that is, indexes of correspondingprecoding matrixes) back to the transmitter on the basis of theaforementioned codebook information. For example, the receiver mayselect an optimized precoding matrix by measuring the received signal inaccordance with a maximum likelihood (ML) scheme or a minimum meansquare error (MMSE) scheme. Although FIG. 10 illustrates that thereceiver transmits precoding matrix information per codeword to thetransmitter, the present invention is not limited to the example of FIG.10.

The transmitter that has received feedback information from the receivermay select a specific precoding matrix from the codebook on the basis ofthe received information. The transmitter that has selected theprecoding matrix may perform precoding in such a way to multiply layersignals equivalent to transmission ranks by the selected precodingmatrix, and may transmit the precoded signals through a plurality ofantennas. The transmitter may notify the receiver what precodinginformation applied to the transmitting signals is. The number of rowsin the precoding matrix is the same as the number of antennas, and thenumber of columns is the same as the rank value. Since the rank value isthe same as the number of layers, the number of columns is the same asthe number of layers. For example, if the number of transmittingantennas is 4 and the number of transmission layers is 2, the precodingmatrix may be configured as a 4×2 matrix. Information transmittedthrough each layer may be mapped into each antenna through the precodingmatrix.

The receiver that has received the signal precoded by and transmittedfrom the transmitter may perform inverse processing of precodingperformed by the transmitter and recover the received signals.Generally, since the precoding matrix satisfies a unitary matrix (U)condition such as U*U^(H)=I, the inverse processing of precoding may beperformed in such a manner that a Hermit matrix P^(H) of the precodingmatrix P used for precoding of the transmitter is multiplied by thereceived signals.

For example, the following Table 4 illustrates a codebook used fordownlink transmission that two transmitting antennas are used in the3GPP LTE release-8/9, and the following Table 5 illustrates a codebookused for downlink transmission that four transmitting antennas are usedin the 3GPP LTE release-8/9.

TABLE 4 Number of rank Codebook index 1 2 0$\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}$ $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 2 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 3 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}$ —

TABLE 5 Codebook Number of layers ν index u_(n) 1 2 3 4 0 u₀ = [1 −1−1]^(T) W₀ ^({1}) W₀ ^({14})/{square root over (2)} W₀ ^({124})/{squareroot over (3)} W₀ ^({1234})/2 1 u₁ = [1 −j 1 j]^(T) W₁ ^({1}) W₁^({12})/{square root over (2)} W₁ ^({123})/{square root over (3)} W₁^({1234})/2 2 u₂ = [1 1 −1 1]^(T) W₂ ^({1}) W₂ ^({12})/{square root over(2)} W₂ ^({123})/{square root over (3)} W₂ ^({3214})/2 3 u₃ = [1 j 1−j]^(T) W₃ ^({1}) W₃ ^({12})/{square root over (2)} W₃ ^({123})/{squareroot over (3)} W₃ ^({3214})/2 4 u₄ = [1 (−1 − j)/{square root over (2)}−j (1 − j)/{square root over (2)}]^(T) W₄ ^({1}) W₄ ^({14})/{square rootover (2)} W₄ ^({124})/{square root over (3)} W₄ ^({1234})/2 5 u₅ = [1 (1− j)/{square root over (2)}j (−1 − j)/{square root over (2)}]^(T) W₅^({1}) W₅ ^({14})/{square root over (2)} W₅ ^({124})/{square root over(3)} W₅ ^({1234})/2 6 u₆ = [1 (1 + j)/{square root over (2)}− j (−1 +j)/{square root over (2)}]^(T) W₆ ^({1}) W₆ ^({13})/{square root over(2)} W₆ ^({134})/{square root over (3)} W₆ ^({1324})/2 7 u₇ = [1 (−1 +j)/{square root over (2)}j (1 + j)/{square root over (2)}]^(T) W₇ ^({1})W₇ ^({13})/{square root over (2)} W₇ ^({134})/{square root over (3)} W₇^({1324})/2 8 u₈ = [1 −1 1 1]^(T) W₈ ^({1}) W₈ ^({12})/{square root over(2)} W₈ ^({124})/{square root over (3)} W₈ ^({1234})/2 9 u₉ = [1 −j −1−j]^(T) W₉ ^({1}) W₉ ^({14})/{square root over (2)} W₉ ^({134})/{squareroot over (3)} W₉ ^({1234})/2 10 u₁₀ = [1 1 1 −1]^(T) W₁₀ ^({1}) W₁₀^({13})/{square root over (2)} W₁₀ ^({123})/{square root over (3)} W₁₀^({1324})/2 11 u₁₁ = [1 j −1 j]^(T) W₁₁ ^({1}) W₁₁ ^({13})/{square rootover (2)} W₁₁ ^({134})/{square root over (3)} W₁₁ ^({1324})/2 12 u₁₂ =[1 −1 −1 1]^(T) W₁₂ ^({1}) W₁₂ ^({12})/{square root over (2)} W₁₂^({123})/{square root over (3)} W₁₂ ^({1234})/2 13 u₁₃ = [1 −1 1 −1]^(T)W₁₃ ^({1}) W₁₃ ^({13})/{square root over (2)} W₁₃ ^({123})/{square rootover (3)} W₁₃ ^({1324})/2 14 u₁₄ = [1 1 −1 −1]^(T) W₁₄ ^({1}) W₁₄^({13})/{square root over (2)} W₁₄ ^({123})/{square root over (3)} W₁₄^({3214})/2 15 u₁₅ = [1 1 1 1]^(T) W₁₅ ^({1}) W₁₅ ^({12})/{square rootover (2)} W₁₅ ^({123})/{square root over (3)} W₁₅ ^({1234})/2

In Table 5, W_(n) ^({s}) is obtained by a set {s} configured fromEquation expressed as W_(n)=I−2u_(n)u_(n) ^(H)/u_(n) ^(H)u_(n). In thiscase, I represents a 4×4 single matrix, and u_(n) is a value given byTable 5.

As illustrated in Table 4, a codebook for two transmitting antennas hasa total of seven precoding vectors/matrixes. In this case, since thesingle matrix is intended for the open-loop system, a total of sixprecoding vectors/matrixes are obtained for precoding of the closed-loopsystem. Also, a codebook for four transmitting antennas as illustratedin Table 5 has a total of sixty-four precoding vectors/matrixes.

Additionally, in the system (for example, 3GPP LTE release-10 oradvanced system) that supports extended antenna configuration, forexample, MIMO transmission based on eight transmitting antennas may beperformed. A codebook design for supporting MIMO transmission isrequired.

For CSI report for the channel transmitted through eight antenna ports,it may be considered that codebooks as illustrated in Table 6 to Table13 are used. Eight CSI-RS antenna ports may be expressed as antenna portindexes 15 to 22. Each of Tables 6, 7, 8, 9, 10, 11, 12 and 13illustrates an example of a codebook for 1-layer, 2-layer, 3-layer,4-layer, 5-layer, 6-layer, 7-layer, and 8-layer CSI reports based on theantenna ports 15 to 22.

In Table 6 to Table 13, φ_(n) and v_(m) may be given by the followingEquation 13.

φ_(n) =e ^(jπn/2)

v _(m)=[1e ^(j2π/32) e ^(j4πm/32) e ^(j6πm/32)]^(T)

TABLE 6 i₂ i₁ 0 1 2 3 4 5 6 7 0-15 W_(2i) ₁ _(,0) ⁽¹⁾ W_(2i) ₁ _(,1) ⁽¹⁾W_(2i) ₁ _(,2) ⁽¹⁾ W_(2i) ₁ _(,3) ⁽¹⁾ W_(2i) ₁ _(+1,0) ⁽¹⁾ W_(2i) ₁_(+1,1) ⁽¹⁾ W_(2i) ₁ _(+1,2) ⁽¹⁾ W_(2i) ₁ _(+1,3) ⁽¹⁾ i₂ i₁ 8 9 10 11 1213 14 15 0-15 W_(2i) ₁ _(+2,0) ⁽¹⁾ W_(2i) ₁ _(+2,1) ⁽¹⁾ W_(2i) ₁ _(+2,2)⁽¹⁾ W_(2i) ₁ _(+2,3) ⁽¹⁾ W_(2i) ₁ _(+3,0) ⁽¹⁾ W_(2i) ₁ _(+3,1) ⁽¹⁾W_(2i) ₁ _(+3,2) ⁽¹⁾ W_(2i) ₁ _(+3,3) ⁽¹⁾${{where}\mspace{14mu} W_{m,n}^{(1)}} = {\frac{1}{\sqrt{8}}\begin{bmatrix}\nu_{m} \\{\phi_{n}\nu_{m}}\end{bmatrix}}$

TABLE 7 i₂ i₁ 0 1 2 3 0-15 W_(2i) ₁ _(,2i) ₁ _(,0) ⁽²⁾ W_(2i) ₁ _(,2i) ₁_(,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+1,0) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+1,1)⁽²⁾ i₂ i₁ 4 5 6 7 0-15 W_(2i) ₁ _(+2,2i) ₁ _(+2,0) ⁽²⁾ W_(2i) ₁ _(+2,2i)₁ _(+2,1) ⁽²⁾ W_(2i) ₁ _(+3,2i) ₁ _(+3,0) ⁽²⁾ W_(2i) ₁ _(+3,2i) ₁_(+3,1) ⁽²⁾ i₂ i₁ 8 9 10 11 0-15 W_(2i) ₁ _(,2i) ₁ _(+1,0) ⁽²⁾ W_(2i) ₁_(,2i) ₁ _(+1,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+2,0) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁_(+2,1) ⁽²⁾ i₂ i₁ 12 13 14 15 0-15 W_(2i) ₁ _(,2i) ₁ _(+3,0) ⁽²⁾ W_(2i)₁ _(,2i) ₁ _(+3,1) ⁽²⁾ W_(2i) ₁ _(+1,2i) ₁ _(+3,0) ⁽²⁾ W_(2i) ₁ _(+1,2i)₁ _(+3,1) ⁽²⁾${{where}\mspace{14mu} W_{m,m^{\prime},n}^{(2)}} = {\frac{1}{4}\begin{bmatrix}\nu_{m} & \nu_{m^{\prime}} \\{\phi_{n}\nu_{m}} & {{- \phi_{n}}\nu_{m^{\prime}}}\end{bmatrix}}$

TABLE 8 i₂ i₁ 0 1 2 3 0-3 W_(8i) ₁ _(,8i) ₁ _(,8i) ₁ ₊₈ ⁽³⁾ W_(8i) ₁_(+8,8i) ₁ _(,8i) ₁ ₊₈ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(,8i) ₁ _(+8,8i) ₁₊₈ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+8,8i) ₁ _(,8i) ₁ ⁽³⁾ i₂ i₁ 4 5 6 7 0-3W_(8i) ₁ _(+2,8i) ₁ _(+2,4i) ₁ ₊₁₀ ⁽³⁾ W_(8i) ₁ _(+10,8i) ₁ _(+2,8i) ₁₊₁₀ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+2,8i) ₁ _(+10,8i) ₁ ₊₁₀ ⁽³⁾ {tildeover (W)}_(8i) ₁ _(+10,8i) ₁ _(+2,8i) ₁ ₊₂ ⁽³⁾ i₂ i₁ 8 9 10 11 0-3W_(8i) ₁ _(+4,8i) ₁ _(+4,8i) ₁ ₊₁₂ ⁽³⁾ W_(8i) ₁ _(+12,8i) ₁ _(+4,8i) ₁₊₁₂ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+4,8i) ₁ _(+12,8i) ₁ ₊₁₂ ⁽³⁾ {tildeover (W)}_(8i) ₁ _(+12,8i) ₁ _(+4,8i) ₁ ₊₄ ⁽³⁾ i₂ i₁ 12 13 14 15 0-3W_(8i) ₁ _(+6,8i) ₁ _(+6,8i) ₁ ₊₁₄ ⁽³⁾ W_(8i) ₁ _(+14,8i) ₁ _(+6,8i) ₁₊₁₄ ⁽³⁾ {tilde over (W)}_(8i) ₁ _(+6,8i) ₁ _(+14,8i) ₁ ₊₁₄ ⁽³⁾ {tildeover (W)}_(8i) ₁ _(+14,8i) ₁ _(+6,8i) ₁ ₊₆ ⁽³⁾${{{where}\mspace{14mu} W_{m,m^{\prime},m^{''}}^{(3)}} = {\frac{1}{\sqrt{24}}\begin{bmatrix}\nu_{m} & \nu_{m^{\prime}} & \nu_{m^{''}} \\\nu_{m} & {- \nu_{m^{\prime}}} & {- \nu_{m^{''}}}\end{bmatrix}}},{{\overset{\sim}{W}}_{m,m^{\prime},m^{''}}^{(3)} = {\frac{1}{\sqrt{24}}\begin{bmatrix}\nu_{m} & \nu_{m^{\prime}} & \nu_{m^{''}} \\\nu_{m} & \nu_{m^{\prime}} & {- \nu_{m^{''}}}\end{bmatrix}}}$

TABLE 9 i₂ i₁ 0 1 2 3 0-3 W_(8i) ₁ _(,8i) ₁ _(+8,0) ⁽⁴⁾ W_(8i) ₁ _(,8i)₁ _(+8,1) ⁽⁴⁾ W_(8i) ₁ _(+2,8i) ₁ _(+10,0) ⁽⁴⁾ W_(8i) ₁ _(+2,8i) ₁_(+10,1) ⁽⁴⁾ i₂ i₁ 4 5 6 7 0-3 W_(8i) ₁ _(+4,8i) ₁ _(+12,0) ⁽⁴⁾ W_(8i) ₁_(+4,8i) ₁ _(+12,1) ⁽⁴⁾ W_(8i) ₁ _(+6,8i) ₁ _(+14,0) ⁽⁴⁾ W_(8i) ₁_(+6,8i) ₁ _(+14,1) ⁽⁴⁾${{where}\mspace{14mu} W_{m,m^{\prime},n}^{(4)}} = {\frac{1}{\sqrt{32}}\begin{bmatrix}\nu_{m} & \nu_{m^{\prime}} & \nu_{m} & \nu_{m^{\prime}} \\{\phi_{n}\nu_{m}} & {\phi_{n}\nu_{m^{\prime}}} & {{- \phi_{n}}\nu_{m}} & {{- \phi_{n}}\nu_{m^{\prime}}}\end{bmatrix}}$

TABLE 10 i₂ i₁ 0 0-3$W_{i_{1}}^{(5)} = {\frac{1}{\sqrt{40}}\begin{bmatrix}\nu_{2i_{1}} & \nu_{2i_{1}} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 16} \\\nu_{2i_{1}} & {- \nu_{2i_{1}}} & \nu_{{2i_{1}} + 8} & {- \nu_{{2i_{1}} + 8}} & \nu_{{2i_{1}} + 16}\end{bmatrix}}$

TABLE 11 i₂ i₁ 0 0-3$W_{i_{1}}^{(6)} = {\frac{1}{\sqrt{48}}\begin{bmatrix}\nu_{2i_{1}} & \nu_{2i_{1}} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 16} & \nu_{{2i_{1}} + 16} \\\nu_{2i_{1}} & {- \nu_{2i_{1}}} & \nu_{{2i_{1}} + 8} & {- \nu_{{2i_{1}} + 8}} & \nu_{{2i_{1}} + 16} & {- \nu_{{2i_{1}} + 16}}\end{bmatrix}}$

TABLE 12 i₂ i₁ 0 0-3$W_{i_{1}}^{(7)} = {\frac{1}{\sqrt{56}}\begin{bmatrix}\nu_{2i_{1}} & \nu_{2i_{1}} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 16} & \nu_{{2i_{1}} + 16} & \nu_{{2i_{1}} + 24} \\\nu_{2i_{1}} & {- \nu_{2i_{1}}} & \nu_{{2i_{1}} + 8} & {- \nu_{{2i_{1}} + 8}} & \nu_{{2i_{1}} + 16} & {- \nu_{{2i_{1}} + 16}} & \nu_{{2i_{1}} + 24}\end{bmatrix}}$

TABLE 13 i₂ i₁ 0 0 $W_{i_{1}}^{(8)} = {\frac{1}{8}\begin{bmatrix}\nu_{2i_{1}} & \nu_{2i_{1}} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 8} & \nu_{{2i_{1}} + 16} & \nu_{{2i_{1}} + 16} & \nu_{{2i_{1}} + 24} & \nu_{{2i_{1}} + 24} \\\nu_{2i_{1}} & {- \nu_{2i_{1}}} & \nu_{{2i_{1}} + 8} & {- \nu_{{2i_{1}} + 8}} & \nu_{{2i_{1}} + 16} & {- \nu_{{2i_{1}} + 16}} & \nu_{{2i_{1}} + 24} & {- \nu_{{2i_{1}} + 24}}\end{bmatrix}}$

Multi-Antenna Array

FIG. 11 is a diagram illustrating examples constituting 8 transmittingantennas.

FIG. 11( a) illustrates that N number of antennas configures mutualindependent channels without grouping. Generally, this antennaconfiguration will be referred to as ULA (Uniform Linear Array).

FIG. 11( b) illustrates ULA type antenna configuration of a pair ofantennas (Paired ULA). In this case, the pair of antennas may have anassociated channel therebetween and have an independent channel fromanother pair of antennas.

If a plurality of transmitting antennas should be installed in aninsufficient space, ULA antenna configuration as illustrated in FIGS.11( a) and 11(b) may not be appropriate. Accordingly, it may beconsidered that dual-pole (or cross-pole) antenna configuration asillustrated in FIG. 11( c) is used. If the transmitting antennas areconfigured in this way, although a distance d between the antennas isrelatively short, an independent channel may be configured by loweringantenna correlation, whereby data transmission of high throughput may beperformed.

In the example of FIG. 11( c), in array of a total of N_(T) number oftransmitting antennas, a group 1 of indexes 1, 2, . . . , N_(T)/2 and agroup 2 of indexes N_(T)/2+1, N_(T)/2+2, . . . , N_(T) may be configuredto have polarizations orthogonal to each other. The antennas of theantenna group 1 may have the same polarization (for example, verticalpolarization) and the antennas of the antenna group 2 may have anothersame polarization (for example, horizontal polarization). Also, the twoantenna groups are co-located. For example, antenna 1 and N_(T)/2+1,antenna 2 and N_(T)/2+2, antenna 3 and N_(T)/2+3, . . . , antennaN_(T)/2 and N_(T) may be co-located. In other words, the antennas withinone antenna group have the same polarization like ULA (Uniform LinearArray), and correlation between the antennas within one antenna grouphas linear phase increment property. Also, correlation between theantenna groups has phase rotation property.

1-Dimensional Antenna Array and CSI Feedback

1-dimensional antenna array may include ULA or cross-pole antenna arrayconfiguration as illustrated in FIG. 11. If this 1-dimensional antennaarray is used, the aforementioned reference signal transmission and CSIfeedback method is used. In other words, in order to estimate thechannel between the transmitter and the receiver (or base station anduser equipment) in downlink transmission, the transmitter may transmitthe reference signal (for example, CRS or CSI-RS) to the receiver, andthe receiver may estimate the channel status based on the referencesignal. The receiver may calculate rank, precoding weight value, and CQIbased on the precoding weight value, which will be expected to besuitable for downlink data transmission, on the basis the channelinformation acquired through the reference signal.

For MIMO transmission such as precoded spatial multiplexing, precodinginformation may be required, wherein the precoding weight value may beconfigured in a type of codebook.

For example, in the MIMO system that uses four transmitting antennas(hereinafter, referred to as 4Tx), CSI feedback for precoded spatialmultiplexing (SM) based on the CRS may be described as follows. When thebase station having four transmitting antennas transmits the CRS, if itis assumed that indexes of antenna ports (AP) mapped into each RS areAP0, 1, 2 and 3, the user equipment may estimate the channels from AP0,1, 2 and 3 by using the CRS.

In this case, if the matrix (or vector) that expresses the channelestimated by the user equipment is H, H=[H₁₁ H₁₂ H₁₃ H₁₄; H₂₁ H₂₂ H₂₃H₂₄; . . . ; H_(Nr1) H_(Nr2) H_(Nr3) H_(Nr4)] may be expressed. In otherwords, H may be expressed as Nr×Nt sized matrix (or vector). In thiscase, Nr is the number of receiving antennas, and Nt is the number oftransmitting antennas.

Also, the user equipment may assume that the base station transmits databy using a precoding weight matrix (or vector) W_(m)(k). In W_(m)(k), mmeans a transmission rank, and k means index of the precoding weightmatrix (or vector) defined for Rank-m. W_(m)(k) may be expressed asW_(m)(k)=[W₁₁ W₁₂ W₁₃ . . . W_(1m); W₂₁ W₂₂ W₂₃ . . . W_(2m); W₃₁ W₃₂W₃₃ . . . W_(3m); . . . ; W₄₁ W₄₂ W₄₃ . . . W_(4m)]. That is, W_(m)(k)may be expressed as Nt×m sized matrix (or vector).

Also, the user equipment may calculate an equivalent channel H_(eq). Theequivalent channel H_(eq) may be calculated by synthesis of theestimated channel H and the precoding weight value W_(m)(k) (that is,H_(eq)=HW_(m)(k)), or may be calculated by synthesis of a CovarianceMatrix R of the estimated channel and the precoding weight valueW_(m)(k) (that is, H_(eq)=RW_(m)(k)). The user equipment may select rankand precoding weight value, which are suitable for downlinktransmission, on the basis of the equivalent channel H_(eq). Also, theuser equipment may calculate CQI expected when the selected rank andprecoding weight value are used.

For another example, in the MIMO system that uses eight transmittingantennas (hereinafter, referred to as 8Tx), CSI feedback for precodedspatial multiplexing (SM) based on the CSI-RS may be described asfollows. When the base station having eight transmitting antennastransmits the CSI-RS, if it is assumed that indexes of antenna ports(AP) mapped into each RS are AP15, 16, 17, 18, 19, 20, 21, 22, the userequipment may estimate the channels from AP15, 16, 17, 18, 19, 20, 21,22 by using the CSI-RS.

In this case, if the matrix (or vector) that expresses the channelestimated by the user equipment is H, H=[H₁₁ H₁₂ H₁₃ H₁₄ H₁₅ H₁₆ H₁₇H₁₈; H₂₁ H₂₂ H₂₃ H₂₄ H₂₅ H₂₆ H₂₇ H₂₈; . . . ; H_(Nr1) H_(Nr2) H_(Nr3)H_(Nr4) H_(Nr5) H_(Nr6) H_(Nr7) H_(Nr8)] (wherein Nr is the number ofreceiving antennas) may be expressed.

Also, the user equipment may assume that the base station transmits databy using a precoding weight matrix (or vector) W_(m)(k). W_(m)(k) may beexpressed as W_(m)(k)=[W₁₁ W₁₂ W₁₃ . . . W_(1m); W₂₁ W₂₂ W₂₃ . . .W_(2m); W₃₁ W₃₂ W₃₃ . . . W_(3m), . . . ; W₈₁ W₈₂ W₈₃ . . . W_(8m)].

Also, the user equipment may select rank and precoding weight value,which are suitable for downlink transmission, on the basis of theequivalent channel H_(eq) (wherein, the equivalent channel is calculatedby H_(eq)=HW_(m)(k) or H_(eq)=RW_(m)(k)), and may calculate CQI expectedwhen the selected rank and precoding weight value are used.

Accordingly, in the MIMO system that supports Nt number of transmittingantennas, the user equipment may feed CSI (for example, RI, PMI, CQI),which is selected/calculated using the CRS or CSI-RS as described above,back to the base station. The base station may determine rank, precodingweight value, and modulation and coding scheme, which are suitable fordownlink transmission, by considering the CSI reported by the userequipment.

2-Dimensional Antenna Array and CSI Feedback

FIG. 12( a) illustrates ULA which is an example of 1-dimensional antennaconfiguration, and FIG. 12( b) illustrates URA (Uniform RectangularArray) which is an example of 2-dimensional antenna configuration.

In the example of ULA of FIG. 12( a), N number of antennas are arrangedat an interval of d_(r). A weave for ULA may be expressed as a vectork_(p). ω_(p) represents a direction of the vector k_(p), and correspondsto an azimuth angle on a x-y plane.

A steering vector represents a set of phase delays suffered by the wave,wherein the set of phase delays is determined by antennas belonging tothe antenna array. If the steering vector is a_(r), the followingEquation may be expressed.

$\begin{matrix}{\mspace{79mu} {{\phi_{p} = {\frac{d_{r}}{\lambda}{\cos ( \psi_{p} )}}}{{a_{r}( \phi_{p} )} = \begin{bmatrix}1 & ^{{- j}\; 2\; \pi \; \phi_{p}} & ^{{- j}\; 2\; \pi \; 2\; \phi_{p}} & \ldots & ^{{- j}\; 2\; {\pi {({N - 1})}}\phi_{p}}\end{bmatrix}^{T}}}} & \lbrack {{Equation}\mspace{14mu} 14} \rbrack\end{matrix}$

In the above Equation 14, λ represents a wavelength. The steering vectora_(r) is defined by N×1 sized complex vector, and each of N number ofelements of a_(r) represents relative phase at each of the antennas ofULA.

In the example of URA of FIG. 12( b), the antennas are arranged2-dimensionally on a x-z plane. URA may be referred to as UPA (UniformPlanar Array). This 2-dimensional antenna structure is suggested as amethod for arranging so many antennas, and may be used for massive MIMOfor maximizing advantages of the existing MIMO technology.

URA of FIG. 12( b) includes N×M number of antennas. The N×M number ofantennas are arranged on an x axis at an interval of d_(r), and on a zaxis at an interval of d_(c). A direction of a wave vector k_(p) of URAmay be expressed by an azimuth angle ψ_(p) on a x-y plane and anelevation angle θ_(p) on a y-z plane. Also, a steering matrix for URAmay be expressed by the following Equation.

$\begin{matrix}{\mspace{79mu} {{\theta_{p} = {\frac{d_{c}}{\lambda}{\sin ( \vartheta_{p} )}}}\mspace{79mu} {\phi_{p} = {\frac{d_{r}}{\lambda}{\cos ( \psi_{p} )}{\cos ( \vartheta_{p} )}}}{{a_{c}( \theta_{p} )} = \begin{bmatrix}1 & ^{{- j}\; 2\; \pi \; \theta_{p}} & ^{{- j}\; 2\; \pi \; 2\; \theta_{p}} & \ldots & ^{{- j}\; 2\; {\pi {({M - 1})}}\theta_{p}}\end{bmatrix}^{T}}{{a_{r}( \phi_{p} )} = \begin{bmatrix}1 & ^{{- j}\; 2\; \pi \; \phi_{p}} & ^{{- j}\; 2\; \pi \; 2\; \phi_{p}} & \ldots & ^{{- j}\; 2\; {\pi {({N - 1})}}\phi_{p}}\end{bmatrix}^{T}}\mspace{79mu} {{A( {\theta_{p},\phi_{p}} )} = {{a_{c}( \theta_{p} )} \cdot {a_{r}( \phi_{p} )}^{T}}}}} & \lbrack {{Equation}\mspace{14mu} 15} \rbrack\end{matrix}$

In the above Equation 15, A(θ_(p),φ_(p)) represents a steering matrix.The steering matrix A(θ_(p),φ_(p)) is defined as N×M sized complexmatrix, and each of N×M number of elements represents relative phase ateach of the antennas of URA.

A beam formed by the 1-dimensional antenna structure like the existingULA is specified by the azimuth angle (for example, horizontal domain)only and cannot be specified by the elevation angle (for example,vertical domain), whereby 2-dimensional beamforming is only supported.This 1-dimensional antenna structure (for example, ULA or cross-polearray configuration) may support adaptive beamforming or spatialmultiplexing in a direction of the azimuth angle, and MIMO transmissionand reception scheme for the 1-dimensional antenna structure is onlydesigned in the existing wireless communication system (for example,system based on 3GPP LTE release-8, 9, 10 and 11).

In the meantime, if 2-dimensional antenna structure (for example, URA)based MIMO transmission and reception scheme, which is intended toimprove system throughput, is supported, the beam formed by the2-dimensional antenna structure may be specified in the direction of theazimuth angle and the direction of the elevation angle, whereby3-dimensional beamforming may be performed.

FIG. 13 is a diagram illustrating examples of beamforming based on2-dimensional antenna configuration.

FIG. 13( a) illustrates examples of sector specific beamforming formedby restricting a certain range of the azimuth angle and a certain rangeof the elevation angle. FIG. 13( b) illustrates examples of UE-specificbeamforming formed by varying the elevation angle on the same azimuthangle.

According to the function of forming the beam by specifying the azimuthangle and the elevation angle as described above, sector specificelevation beamforming (for example, vertical pattern beamwidth and/ordowntilt based adaptive control), improved sectorization in the verticaldomain, and new beamforming such as user (or UE)-specific elevationbeamforming may be supported.

Vertical sectorization may increase average system throughput throughgain of a vertical sector pattern, and does not require support ofadditional standard technology.

UE-specific elevation beamforming may improve SINR to the correspondingUE by designating a vertical antenna pattern. On the other hand, unlikevertical sectorization or sector-specific vertical beamforming,UE-specific elevation beamforming requires support of additionalstandard technology. For example, in order to normally support2-dimensional port structure, CSI measurement and feedback method of theUE for UE-specific elevation beamforming will be required.

In order to support UE-specific elevation beamforming, a downlink MIMOimprovement method will be required. Examples of the downlink MIMOimprovement method may include improvement (for example, new codebookdesign, method for supporting codebook selection/update/modification,minimization of CSI payload size increase, etc.) of CSI feedback methodof the UE, change of CSI-RS configuration for UE-specific elevationbeamforming, definition of additional antenna ports for UE-specificelevation beamforming, and improvement (for example, method forobtaining common channel coverage and/or RRM (Radio Resource Management)measurement reliability if the number of antenna ports is increased) ofdownlink control operation for supporting UE-specific elevationbeamforming.

Also, in designing the improved downlink MIMO operation, various factorssuch as base station (eNB) antenna calibration error (error on phase andtime), estimation error, downlink overhead, complexity, feedbackoverhead, backward compatibility, actual UE realization, reuse of theexisting feedback framework, and subband to wideband feedback may beconsidered.

Method for Transmitting Reference Signal to Support 2-DimensionalAntenna Structure

The present invention suggests a method for transmitting a referencesignal and a CSI feedback method to normally and efficiently supportschemes such as UE-specific elevation beamforming and verticalsectorization, which may be performed by the 2-dimensional antennastructure.

In the existing system, a beam direction is fixed in a verticaldirection (that is, vertical direction of beam cannot beselected/adjusted), and horizontal beamforming may be performed. Thebase station command the UE to perform CSI-RS configuration and transmitthe CSI-RS based on the CSI-RS configuration to the UE, so that the UEmay report CSI, which includes PMI, etc., to the base station, therebydetermining the most appropriate horizontal beamforming. The command toperform the CSI-RS configuration means that one or more of information(for example, CSI-RS port, CSI-RS transmission timing, CSI-RStransmission RE position, etc.) included in the CSI-RS-Config IE ofTable 2 is provided.

For 3-dimensional beamforming, vertical beamforming (or selection ofvertical beam) is required additionally to the existing horizontalbeamforming, and a detailed method for additional vertical beamforminghas not been defined yet.

In order to describe the basic principle of the present invention,2-dimensional URA (or UPA) may be assumed by combination of ULA of afirst domain (for example, horizontal domain) and ULA of a second domain(for example, vertical domain). For example, 3-dimensional beam may beformed in such a manner that the azimuth angle is determined in thehorizontal domain after the elevation angle is determined in thevertical domain, or the elevation angle is determined in the verticaldomain after the azimuth angle is determined in the horizontal domain.In this 2-dimensional antenna structure, selection of ULA for any one ofthe first and second domains may be referred to as regional selection ordomain selection.

Also, in 3-dimensional beamforming, one dimension may be determined by astatic or long-term, and the other two dimensions may be determined by adynamic or short-term. For example, a beam of a specific direction inthe vertical domain may be determined and beamforming of the horizontaldomain according to the existing beamforming technology may be performedfor the corresponding vertical beam, whereby the 3-dimensional beam maybe formed.

For example, it is assumed that N number of beamformings may beavailable vertically and M number of beamformings may be availablehorizontally in a planar array that includes K×L number of antennas. Inthis case, one dominant beam of the N number of vertical beam patternsmay be determined, and one beam may be selected from the M number ofhorizontal beam patterns, whereby the 3-dimensional beam may be formed.At this time, if L number of antennas of the horizontal domain areequivalent to the number of antennas of the MIMO system which isconventionally designed (for example, L=2, 4, 8), the existing feedbackcodebook may be used for CSI feedback for horizontal beamforming.

As described above, in the 2-dimensional antenna structure, verticalbeamforming (or elevation beamforming) may be performed together withhorizontal beamforming (or azimuth beamforming). If vertical beamformingis added to the existing horizontal beamforming, a new method for CSIdetermination/calculation (that is, determination of rank and precodingweight value (or precoding matrix) and CQI calculation) and a method fortransmitting a reference signal will be required.

Also, the MIMO system having the 2-dimensional antenna structure mayperform vertical sectorization and at the same time perform horizontalbeamforming To this end, the method for transmitting a reference signaland a new method for CSI determination/calculation will be required.

In order to describe the method for transmitting a reference signal usedto acquire channel status information in the MIMO system having the2-dimensional antenna structure, the 2-dimensional antenna structure isassumed, in which Ntv number of antennas are arranged in the verticaldomain and Nth number of antennas are arranged in the horizontal domain.In this 2-dimensional antenna structure, the base station has a total ofNt (Nt=Ntv×Nth) number of antennas. In order that the UE estimates thechannel transmitted from the Nt number of antennas of the base station,the base station may allocate and transmit the reference signal to eachof the Nt number of antennas.

FIGS. 14 and 15 are diagrams illustrating a method for allocatingantenna port numbers in a 2-dimensional antenna structure.

FIG. 14 illustrates an example of a total of 16 antennas (that is, 2×8antenna structure) that include 8 columns in a horizontal direction and2 rows in a vertical direction.

FIG. 14( a) illustrates an example of counting antenna port numbers in ahorizontal first manner. In the example of FIG. 14( a), for a total of16 antennas (that is, 2×8 antenna structure) that include 8 columns in ahorizontal direction and 2 rows in a vertical direction, antenna portnumbers p, p+1, p+7 are sequentially allocated to eight antennas in ahorizontal direction of the first row, if there is no antenna to whichantenna port number will be allocated in a horizontal direction, antennaport numbers p+8, p+9, p+15 are sequentially allocated to eight antennasof the second row in a vertical direction.

FIG. 14( b) illustrates an example of counting antenna port numbers in avertical first manner. In the example of FIG. 14( b), antenna portnumbers p, p+1 are sequentially allocated to two antennas in a verticaldirection of the first column, if there is no antenna to which antennaport number will be allocated in a vertical direction, antenna portnumbers p+2, p+3 are sequentially allocated to two antennas of thesecond column in a vertical direction. In this way, the antenna portnumbers are allocated to a total of 16 antennas in accordance with avertical first manner.

FIG. 15 illustrates an example of a total of 16 antennas (that is, 4×4antenna structure) that include 4 columns in a horizontal direction and4 rows in a vertical direction. FIG. 15( a) illustrates an example ofcounting antenna port numbers in a horizontal first manner. FIG. 15( b)illustrates an example of counting antenna port numbers in a verticalfirst manner.

In the MIMO system to which the aforementioned 2-dimensional antennastructure is applied, in order that the receiver determines/calculateschannel status information of the channel formed from the 2-dimensionalantenna structure, the reference signal suitable for the 2-dimensionalantenna structure should be transmitted from the transmitter.Hereinafter, examples of the present invention for reference signalconfiguration suitable for the 2-dimensional antenna structure will bedescribed.

Method 1

According to the method 1 of the present invention, reference signalconfiguration for channel estimation of the 2-dimensional antennastructure may be defined by reuse or modification of the CSI-RSconfiguration defined in the 3GPP LTE release-10 or 11.

The above Table 2 illustrates configuration of the CSI-RS-Config IEdefined in the 3GPP LTE release-10 or 11. For example, in the systemaccording to the release-10, a CSI-RS pattern for supporting 1, 2, 4 or8Tx antenna ports has been defined. As illustrated in FIG. 8, on oneresource block pair, one of 32 patterns may be used for 2Tx antenna portCSI-RS, one of 16 patterns may be used for 4Tx antenna port CSI-RS, andone of 8 patterns may be used for 8Tx antenna port CSI-RS.

Also, as illustrated in Table 1, for configuration for a subframe atwhich the CSI-RS is transmitted, one of 155 combinations of period andoffset for which the CSI-RS is transmitted, may be used.

Also, the CSI-RS is power boosted in accordance with a p_C parametervalue, wherein the same power boosting value is applied to each antennaport.

CSI-RS configuration of the existing 3GPP LTE release-10/11 may be usedby being corrected to CSI-RS configuration for the 2-dimensional antennastructure as follows.

Method 1-1

If the 2-dimensional antenna structure has maximum 8 Tx antennas, CSI-RSconfiguration of the 3GPP LTE release-10/11 may be used by modification.

In other words, CSI-RS resource for the 2-dimensional antenna structuremay be allocated using the CSI-RS configuration of the 3GPP LTErelease-10/11. However, since the CSI-RS configuration of the 3GPP LTErelease-10/11 has been designed for the 1-dimensional antenna structure,if the feedback codebook for the 1-dimensional antenna structure is usedfor the CSI-RS for the 2-dimensional antenna structure, maximization ofthroughput cannot be expected. Accordingly, the feedback codebook forthe 2-dimensional antenna structure should newly be designed.

Also, if the CSI-RS configuration for the 1-dimensional antennastructure and the CSI-RS configuration for the 2-dimensional antennastructure are provided in the same IE type, it is required to identifythe CSI-RS configuration for the 1-dimensional antenna structure and theCSI-RS configuration for the 1-dimensional antenna structure from eachother. For example, if the transmission mode (or 3-dimensionalbeamforming transmission mode) based on the 2-dimensional antennastructure is defined as a new transmission mode (for example, TM11), itmay be indicated explicitly or implicitly that the feedback codebook (orfeedback codebook for 3-dimensional beamforming) for the 2-dimensionalantenna structure should be used if TM11 is set.

The feedback codebook for the 2-dimensional antenna structure may beconfigured similarly to the existing 8Tx codebook (for example, Tables 6to 13) of the 1-dimensional antenna structure. However, since precodingvectors/matrixes suitable for the property of the 2-dimensional antennastructure should be included in the codebook, the existing codebookcannot be used as it is.

First of all, factors constituting the existing 8Tx rank-1 codebook(Table 6) of the 1-dimensional antenna structure are defined to dependon the principle of a precoding vector W as expressed by the followingEquation 16.

$\begin{matrix}{W = \begin{bmatrix}{W\; 1} \\{{aW}\; 1}\end{bmatrix}} & \lbrack {{Equation}\mspace{14mu} 16} \rbrack\end{matrix}$

In the Equation 16, the factor W1 constituting the vector W1 is definedas expressed by the following Equation 17.

$\begin{matrix}{{W\; 1} = {\frac{1}{\sqrt{N_{T}}}^{j\; 2\; \pi \; {{nk}/N}}}} & \lbrack {{Equation}\mspace{14mu} 17} \rbrack\end{matrix}$

In the Equation 17, N_(T) is the number of transmitting antennas. Nmeans the number of beams, n has a value of 0, 1, 2, . . . , N−1, and khas a value of 0, 1, 2, . . . , N_(T)/2−1. For example, the vector W1 ofthe Equation 16 may be a 4×1 sized vector corresponding to (1/√8)×v_(m)of the Equation 13 and Table 6. In W, W1 is applied to antenna indexes1, 2, . . . , N_(T)/2 (that is, pole antennas in FIG. 11( c)) in a1-dimensional cross pole antenna structure of FIG. 11 (c), aW1 isapplied to antenna indexes N_(T)/2+1, N_(T)/2+2, . . . , N_(T) (that is,\\\\ pole antennas in FIG. 11( c)), and phase difference between / poleantenna and \ pole antenna is compensated by a ε{ 1,−1,j,−j}.

FIG. 16 is a diagram illustrating an example of 2-dimensional antennaarray.

In design of the feedback codebook for 2-dimensional antenna array, theprecoding vector W defined as expressed by the following Equation 18 maybe used, wherein the Equation 18 is modified from the Equation 16 whichis the basic Equation used for the 8Tx codebook design of the1-dimensional antenna structure.

$\begin{matrix}{W = \begin{bmatrix}{W\; 1} \\{{aW}\; 2}\end{bmatrix}} & \lbrack {{Equation}\mspace{14mu} 18} \rbrack\end{matrix}$

In case of 1-dimensional cross pole antenna array, the same precodingvector W1 is applied to a first antenna group (antenna indexes 1, 2, . .. , N_(T)/2) and a second antenna group (antenna indexes N_(T)/2+1,N_(T)/2+2, . . . , N_(T)). On the other hand, in case of 2-dimensionalantenna array, W1 is applied to a first antenna group (antenna indexes1, 2, . . . , N_(T)/2), whereas W2 different from W1 is applied to asecond antenna group (antenna indexes N_(T)/2+1, N_(T)/2+2, . . . ,N_(T)). In 2-dimensional antenna array, as the precoding vector appliedto the first antenna group, a value which is not the same as, butsimilar to, the precoding vector applied to the second antenna group maybe used. In more detail, the values of the vectors applied to the firstantenna group and the second antenna group may be modified due toelevational beamforming. For example, W1 and W2 may be defined asexpressed by the following Equation 19.

$\begin{matrix}{{{W\; 1} = {\frac{1}{\sqrt{N_{T}}}^{j\; 2\; \pi \; {{nk}/N}}}}{{W\; 2} = {\frac{1}{\sqrt{N_{T}}}^{j\; 2\; \pi \; {{bnk}/N}}}}} & \lbrack {{Equation}\mspace{14mu} 19} \rbrack\end{matrix}$

As expressed by the above Equation 19, W1 and W2 are different from eachother in that whether a value of b has been used or not. Also, the valueof b is defined by a value determined based on ‘a’ which is a combiningvalue. For example, the value of b may be defined as expressed by thefollowing Equation 20.

b=√{square root over (1−a ²)}  [Equation 20]

The factors (that is, precoding vectors/matrixes) which will be includedin the feedback codebook for supporting the 2-dimensional antennastructure may be configured on the basis of the precoding vector W whichis defined as above. If the transmission mode for supporting the2-dimensional antenna structure is set, the UE may use the precodingvectors/matrixes on the codebook newly defined as above whenselecting/calculating CSI.

Method 1-2

Resource configuration resourceConfig for vertical antennas may beindicated explicitly while CSI-RS configuration is being indicated onthe basis of horizontal antennas.

In more detail, it is defined that a value of antennaPortsCountparameter {an1, an2, an4, an8, . . . } indicated by the CSI-RS-Config IEindicates the number of antennas (for example, L in K×L antenna array)of the horizontal domain in the 2-dimensional antenna array. In case ofthe 2-dimensional antenna array, since the same number of antennas ofthe horizontal domain exist in each row, one antennaPortsCount parameterexists.

Next, the resourceConfig parameter indicated by the CSI-RS-Config IE isset to have one value of ranges (for example, in case of an2, 0 to 31)determined in accordance with the antennaPortsCount value. In this case,a plurality of resourceConfig parameters may be included in the2-dimensional antenna array. For example, if two antennas exist in thevertical domain, index indicating a resource element position (that is,CSI-RS pattern) where antennaPortsCount number of antennas of the firstrow will be arranged may be indicated by resourceConfig A parameter, andindex indicating a resource element position where antennaPortsCountnumber of antennas of the second row will be arranged may be indicatedby resourceConfig B parameter.

As described above, the CSI-RS-Config IE for the 2-dimensional antennaarray may be configured as illustrated in Table 14 below. Table 14corresponds to a part of Table 2, and in Table 14, portions the same asTable 2 are omitted, and modified/added portions from Table 2 are markedwith underlines.

TABLE 14 ... antennaPortsCount-rxx ENUMERATED {an1, an2, an4, an8},resourceConfig-rxx A INTEGER (0..31), resourceConfig-rxx BINTEGER (0..31), subframeConfig-rxx   INTEGER (0..154), p-C-rxx  INTEGER (−8..15) ...

If four antennas exist in the vertical domain, four resourceConfigparameters may be included in the CSI-RS-Config IE. In this way, theresourceConfig parameters equivalent to the number of antennas in thevertical domain may be included in the CSI-RS-Config IE. For example, ifNtv number of antennas exist in the vertical domain, Ntv number ofresourceConfig parameters may be included in the CSI-RS-Config IE. As aresult, the user equipment UE may know that the corresponding CSI-RSconfiguration is for the 2-dimensional antenna array ofNtv×antennaPortsCount number of antennas.

Method 1-3

A parameter for vertical antennas may be indicated additionally whileCSI-RS configuration is being indicated on the basis of horizontalantennas.

It is defined that a value of antennaPortsCount parameter{an1,an2,an4,an8, . . . } indicated by the CSI-RS-Config IE indicatesthe number of antennas (for example, Ntv in Ntv×Nth antenna array) ofthe horizontal domain in the 2-dimensional antenna array. In case of the2-dimensional antenna array, since the same number of antennas of thehorizontal domain exist in each row, one antennaPortsCount parameterexists.

In order to indicate the number of antennas in the vertical domain, aVantennaPortsCount parameter is additionally defined. If the number ofantennas in the vertical domain is indicated by the VantennaPortsCountparameter, resources equivalent to the number of antennas in thevertical domain should be allocated additionally.

The resources which are allocated additionally may be indicatedimplicitly by the resourceConfig parameter indicated by theCSI-RS-Config IE. For example, a predetermined offset value mapped intothe value of the VantennaPortsCount parameter is added to the indexvalue indicated by the resourceConfig parameter, whereby resourcescorresponding to a resultant value may be determined as the resourceswhich are allocated additionally.

As described above, the CSI-RS-Config IE for the 2-dimensional antennaarray may be configured as illustrated in Table 15 below. Table 15corresponds to a part of Table 2, and in Table 15, portions the same asTable 2 are omitted, and modified/added portions from Table 2 are markedwith underlines.

TABLE 15 ... antennaPortsCount-rxx ENUMERATED {an1, an2, an4, an8},resourceConfig-rxx   INTEGER (0..31), VantennaPortsCount-rxx  ENUMERATED {an1, an2, an4, an8}, subframeConfig-rxx   INTEGER(0..154), p-C-rxx   INTEGER (−8..15) ...

According to the example of Table 15, for example, if theantennaPortsCount parameter indicates an4 and the resourceConfigparameter indicates a value of 0, a resource element positioncorresponding to index 0 is selected from CSI-RS resources defined for4Tx. And, if the VantennaPortsCount parameter indicates an2, it may bedetermined that two antennas exist in the vertical domain (that is, 2×4antenna array). It is assumed that an offset value mapped when theVantennaPortsCount parameter is an2 is Offset2. In this case, a resourceelement position (that is, CSI-RS pattern) corresponding to index(0+Offset2) may be selected in addition to the CSI-RS resource elementposition corresponding to index 0. For example, in case of Offset2=1, aCSI-RS resource element position corresponding to index 1 may beselected additionally. Accordingly, a resource element patterncorresponding to index 0 may be determined for CSI-RS transmission forfour horizontal domain antennas of the first row in the vertical domain,and a resource element pattern corresponding to index 1 may bedetermined for CSI-RS transmission for four horizontal domain antennasof the second row in the vertical domain.

Also, if the antennaPortsCount parameter indicates an4 and theVantennaPortsCount parameter indicates an2, the UE may recognize thatthe base station performs 8Tx antenna transmission, and may use atransmission method and/or channel measurement method, which is definedfor 8Tx antenna transmission.

For example, if the antennaPortsCount parameter indicates an4 and theresourceConfig parameter indicates 0, a resource element positioncorresponding to index 0 is selected from the CSI-RS resources definedfor 4Tx. If the VantennaPortsCount parameter indicates an4, it may bedetermined that four antennas exist in the vertical domain (that is, 4×4antenna array). It is assumed that an offset value mapped when theVantennaPortsCount parameter is an24 is Offset4. In this case, aresource element position (that is, CSI-RS pattern) corresponding toindex (0+Offset4) may be selected in addition to the CSI-RS resourceelement position corresponding to index 0. For example, the value ofOffset4 may be 1, 2 and 3. Accordingly, a resource element patterncorresponding to index 0 may be determined for CSI-RS transmission forfour horizontal domain antennas of the first row in the vertical domain,a resource element pattern corresponding to index 1 may be determinedfor CSI-RS transmission for four horizontal domain antennas of thesecond row in the vertical domain, a resource element patterncorresponding to index 2 may be determined for CSI-RS transmission forfour horizontal domain antennas of the third row in the vertical domain,and a resource element pattern corresponding to index 4 may bedetermined for CSI-RS transmission for four horizontal domain antennasof the fourth row in the vertical domain.

Also, if the antennaPortsCount parameter indicates an4 and theVantennaPortsCount parameter indicates an4, the UE may recognize thatthe base station performs 16Tx antenna transmission, and may use atransmission method and/or channel measurement method, which is definedfor 16Tx antenna transmission.

Method 2

For reference signal transmission for channel estimation in the2-dimensional antenna structure, CSI-RS configuration for the horizontaldomain and CSI-RS configuration for the vertical domain may beindicated. That is, CSI-RS configuration may be configured in accordancewith a target or usage of the CSI-RS.

The following Table 16 illustrates an example of configurationinformation on the CSI-RS for vertical domain antennas.

TABLE 16 ... VantennaPortsCount-rxx ENUMERATED {an1, an2, an4, an8},VresourceConfig-rxx INTEGER (0..31), VsubframeConfig-rxx   INTEGER(0..154), p-C-rxx   INTEGER (−8..15) ...

The base station may configure CSI-RS configuration for antennas of thevertical domain as illustrated in Table 16 and notify the UE of theCSI-RS configuration. Accordingly, the UE may receive the CSI-RS basedon the notified CSI-RS configuration and perform channel estimation byusing the received CSI-RS. The channel estimated by the UE is thechannel for antennas of the vertical domain. The UE may select aprecoding vector preferred for the estimated channel and report theselected precoding vector to the base station. The base station mayperform vertical beamforming for the corresponding UE by considering thereported precoding vector.

Also, configuration information on the CSI-RS for horizontal domainantennas may be configured as illustrated in Table 17 below.

TABLE 17 ... HantennaPortsCount-rxx ENUMERATED {an1, an2, an4, an8},HresourceConfig-rxx INTEGER (0..31), HsubframeConfig-rxx   INTEGER(0..154), p-C-rxx INTEGER (−8..15) ...

The base station may configure CSI-RS configuration for antennas of thehorizontal domain as illustrated in Table 17 and notify the UE of theCSI-RS configuration. Accordingly, the UE may receive the CSI-RS basedon the notified CSI-RS configuration and perform channel estimation byusing the received CSI-RS. The channel estimated by the UE is thechannel for antennas of the horizontal domain. The UE may select aprecoding vector preferred for the estimated channel and report theselected precoding vector to the base station. The base station mayperform horizontal beamforming for the corresponding UE by consideringthe reported precoding vector.

In this case, if the UE receives the CSI-RS for antennas of the verticaldomain and reports CSI for vertical beamforming to the base station andthe base station determines vertical beamforming based on the reportedCSI, the base station may notify the UE of CSI-RS configuration forantennas of the horizontal domain by assuming the status based on thedetermined vertical beamforming. In other words, The base stationdetermines optimized horizontal beamforming based on verticalbeamforming without determining horizontal beamforming regardless of adirection of vertical beamforming. That is, the base station may notifythe UE of CSI-RS configuration in the horizontal domain based onvertical beamforming and finally determine horizontal beamforming byconsidering CSI feedback reported from the UE. Accordingly, the basestation may determine final 3-dimensional beamforming considering bothvertical beamforming and horizontal beamforming.

Also, determination of horizontal beamforming based on verticalbeamforming may be applied favorably when CSI-RS configuration and/ortransmission for antennas in the vertical domain is performed by astatic or long-term and CSI-RS configuration and/or transmission forantennas in the horizontal domain is performed by a dynamic orshort-term. That is, CSI-RS configuration for antennas in the verticaldomain may be changed or provided less frequently than CSI-RSconfiguration for antennas in the horizontal domain. Also, the CSI-RSfor antennas in the vertical domain may be transmitted less frequentlythan the CSI-RS for antennas in the horizontal domain. In this case, avalue of CSI-RS subframe period determined by the VsubframConfigparameter of Table 16 may be set to a value greater than that of CSI-RSsubframe period determined by the HsubframConfig parameter of Table 17.

Method 3

For reference signal transmission for channel estimation in the2-dimensional antenna structure, multiple CSI-RS configurations may beconfigured.

Multiple CSI-RS configurations means that beamforming of any one of thehorizontal and vertical domains is determined by the precoding vectorpreviously determined by the base station (that is, beamforming is notdetermined based on CSI feedback of UE), and beamforming of the otherone domain is determined based on the precoding vector selected andreported by the UE in accordance with CSI-RS configuration andtransmission from the base station. In other words, multiple CSI-RSconfigurations for determining beamforming of the second domain may beconfigured in a state that beamforming of the first domain is previouslydetermined.

For example, in case of vertical domain sectorization, multiple CSI-RSconfigurations may be configured for one azimuth angle. In this case,the base station may notify the UE of a plurality of CSI-RSconfigurations for vertical beamforming after determining one azimuthangle (that is, beamforming of horizontal domain is applied inaccordance with a previously determined requirement). The UE maydetermine CSI-RS reception, channel estimation, and precoding vector inaccordance with each of the plurality of CSI-RS configurations. As aresult, a plurality of precoding vectors corresponding to the pluralityof CSI-RS configurations may be determined, and the UE may select apreferred one of the plurality of precoding vectors and report theselected precoding vector to the base station. The base station mayperform vertical beamforming for the plurality of CSI-RS configurationsby considering the precoding vector reported by the UE.

For example, the precoding vector for multiple CSI-RS configurations forone azimuth angle may include eight elements as illustrated in Table 18below. In Table 18, H means a channel value of each of eight antennaports, and h means a channel value for a antenna port set (that is, oneantenna port sent includes four antenna ports) obtained by synthesis oftwo of eight antenna ports. For example, h11, h12, h13 and h14 representchannel values for the first antenna port set that includes four antennaports. Also, h21, h22, h23 and h24 represent channel values for thesecond antenna port set that includes another four antenna ports.Channel properties as illustrated in the example of Table 18 may begenerated when different vertical beams are applied to the first andsecond antenna port sets. The UE may select a preferred one of eightelements and report the selected element to the base station.

TABLE 18 h11 = (H1 + h12 = (H2 + H6) h13 = (H3 + H7) h14 = (H4 + H8) H5)h21 = (H1 − h22 = (H2 − H6) h23 = (H3 − H7) h24 = (H4 − H8) H5)

Alternatively, multiple CSI-RS configurations may be configured for oneelevation angle. In this case, the base station may notify the UE of aplurality of CSI-RS configurations for horizontal beamforming afterdetermining one elevation angle (that is, beamforming of vertical domainis applied in accordance with a previously determined requirement). TheUE may determine CSI-RS reception, channel estimation, CSIselection/calculation and CSI report in accordance with each of theplurality of CSI-RS configurations. As a result, the base station mayselect a proper one of a plurality of CSIs (or a plurality of precodingvectors) reported by the UE for a plurality of CSI-RS configurations andperform horizontal beamforming by using the selected CSI.

FIG. 17 is a flow chart illustrating CSI-RS related operation for a2-dimensional antenna structure according to the present invention.

At step S1710, the base station may configure CSI-RS configurationinformation for the 2-dimensional antenna structure and provide theconfigured CSI-RS configuration information to the base station. TheCSI-RS configuration information may be configured by one or combinationof at least two of the details described in the various embodiments ofthe present invention.

At step S1720, the base station may transmit the CSI-RS for the2-dimensional antenna structure to the user equipment. The userequipment may receive the CSI-RS for the 2-dimensional antenna structureon the basis of the CSI-RS configuration information provided from thebase station at step S1710.

At step S1730, the user equipment may estimate the channel by using thereceived CSI-RS and generate CSI for the channel formed by the2-dimensional antenna structure of the base station. The operation ofgenerating CSI for the 2-dimensional antenna structure of the basestation may be configured by one or combination of at least two of thedetails described in the various embodiments of the present invention.

At step S1740, the user equipment may report the generated CSI to thebase station.

FIG. 18 is a diagram illustrating a base station and a user equipmentaccording the preferred embodiment of the present invention.

Referring to FIG. 18, a base station 10 according to the presentinvention may include a transmitter 11, a receiver 12, a processor 13, amemory 14, and a plurality of antennas 15. The transmitter 11 maytransmit various signals, data and information to an external device(for example, user equipment). The receiver 10 may receive varioussignals, data and information from the external device (for example,user equipment). The processor 13 may control the overall operation ofthe base station 10. The plurality of antennas 15 may be configured inaccordance with the 2-dimensional antenna structure.

The processor 13 of the base station 10 according to the embodiment ofthe present invention may be configured to configure CSI-RSconfiguration information, which will be provided to the user equipment,in accordance with the embodiments suggested in the present invention,transmit CSI-RS on the basis of the CSI-RS configuration information,and receive CSI generated by the user equipment. In addition, theprocessor 13 of the base station 10 performs a function ofoperation-processing information received by the base station 10 andinformation to be transmitted to the external device. The memory 14 maystore the operation-processed information for a predetermined time, andmay be replaced with a buffer (not shown).

Referring to FIG. 18, a user equipment 20 according to the presentinvention may include a transmitter 21, a receiver 22, a processor 23, amemory 24, and a plurality of antennas 25. The plurality of antennas 25mean user equipments that support MIMO transmission and reception. Thetransmitter 21 may transmit various signals, data and information to anexternal device (for example, base station). The receiver 20 may receivevarious signals, data and information from the external device (forexample, base station). The processor 23 may control the overalloperation of the user equipment 20.

The processor 23 of the user equipment 20 according to the embodiment ofthe present invention may be configured to receive CSI-RS on the basisof CSI-RS configuration information provided from the base station inaccordance with the embodiments suggested in the present invention, andgenerate and report CSI for the 2-dimensional antenna structure of thebase station by using the received CSI-RS. In addition, the processor 23of the user equipment 20 performs a function of operation-processinginformation received by the user equipment 20 and information to betransmitted to the external device. The memory 24 may store theoperation-processed information for a predetermined time, and may bereplaced with a buffer (not shown).

The details of the aforementioned user equipment 20 may be configured insuch a manner that the aforementioned various embodiments of the presentinvention may independently be applied thereto, or two or moreembodiments may simultaneously be applied thereto. The repeateddescription of the details of the user equipment 20 will be omitted forclarification.

Also, in the description of the various embodiments of the presentinvention, the base station has been described as a downlinktransmission entity or uplink reception entity, and the user equipmenthas been described as a downlink reception entity or uplink transmissionentity. However, the scope of the present invention is not limited tothe above example. For example, the description of the base station mayequally be applied to a case where a cell, an antenna port, an antennaport group, RRH, a transmission point, a reception point, an accesspoint or a relay becomes a downlink transmission entity to the userequipment or an uplink reception entity from the user equipment. Also,the principle of the present invention described through the variousembodiments of the present invention may equally be applied to even acase where the relay becomes a downlink transmission entity to the userequipment or an uplink reception entity from the user equipment, or acase where the relay becomes an uplink transmission entity to the basestation or a downlink reception entity from the base station.

The aforementioned embodiments according to the present invention may beimplemented by various means, for example, hardware, firmware, software,or their combination.

If the embodiments according to the present invention are implemented byhardware, the embodiments of the present invention may be implemented byone or more application specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

If the embodiments according to the present invention are implemented byfirmware or software, the embodiments of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

It will be apparent to those skilled in the art that the presentinvention may be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is also obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by a subsequent amendment after theapplication is filed.

INDUSTRIAL APPLICABILITY

The aforementioned embodiments according to the present invention may beapplied to various wireless communication systems.

1. A method for reporting channel state information (CSI) from a userequipment in a wireless communication system, the method comprising:receiving a channel state information-reference signal (CSI-RS) based onCSI-RS configuration information provided by a base station; andreporting to the base station the CSI generated using the CSI-RS,wherein the CSI includes precoding information selected from apredetermined codebook, elements of the predetermined codebook areconfigured based on a precoding vector W, the precoding vector W is${W = \begin{bmatrix}{W\; 1} \\{{aW}\; 2}\end{bmatrix}},$ W1 is a precoding vector applied to a first domainantenna group having a 2D antenna structure, W2 is a precoding vectorapplied to a second domain antenna group having the 2D antennastructure, and “a” is a value representing phase difference between thefirst domain antenna group and the second domain antenna group.
 2. Themethod according to claim 1, wherein${{W\; 1} = {\frac{1}{ \sqrt{}N_{T} }^{j\; 2\; \pi \; {{nk}/N}}}},{{W\; 2} = {\frac{1}{\sqrt{N_{T}}}^{j\; 2\; \pi \; {{bnk}/N}}}},$N_(T) is the number of transmitting antennas, n=0, 1, 2, . . . , N−1,k=0, 1, 2, . . . , N_(T)/2−1, and N is the number of beams.
 3. Themethod according to claim 2, wherein “b” is determined based on “a”. 4.The method according to claim 2, wherein b=√{square root over (1−a²)}.5. The method according to claim 1, wherein the CSI-RS configurationinformation includes an antenna port count parameter and Ntv number ofresource configuration parameters, the antenna port count parameterindicates the number of antennas of the first domain antenna group, andthe Ntv corresponds to the number of antennas of the second domainantenna group.
 6. The method according to claim 1, wherein the CSI-RSconfiguration information includes a first antenna port count parameter,a second antenna port count parameter and a resource configurationparameter, the first antenna port count parameter indicates the numberof antennas of the first domain antenna group, the second antenna portcount parameter indicates the number of antennas of the second domainantenna group, and the resource configuration parameter indicates aresource element location of the CSI-RS for the first domain antennagroup corresponding to one antenna of the second domain antenna group.7. The method according to claim 6, wherein a resource element locationof the CSI-RS for the first domain antenna group corresponding to eachof the other antennas of the second domain antenna group is determinedbased on an offset value corresponding to a value of the second antennaport count parameter.
 8. The method according to claim 1, wherein the 2Dantenna structure is configured by the number of antennas of the seconddomain antenna group×the number of antennas of the first domain antennagroup.
 9. The method according to claim 1, wherein the first domain is ahorizontal domain, and the second domain is a vertical domain.
 10. Auser equipment for reporting channel state information (CSI) in awireless communication system, the user equipment comprising: areceiver; a transmitter; and a processor, wherein the processor isconfigured to: control the receiver to receive a channel stateinformation-reference signal (CSI-RS) based on CSI-RS configurationinformation that is provided by a base station, and control thetransmitter to report to the base station the CSI generated using theCSI-RS, the CSI includes precoding information selected from apredetermined codebook, elements of the predetermined codebook areconfigured based on a precoding vector W, the precoding vector W is${W = \begin{bmatrix}{W\; 1} \\{{aW}\; 2}\end{bmatrix}},$ W1 is a precoding vector applied to a first domainantenna group having a 2D antenna structure, W2 is a precoding vectorapplied to a second domain antenna group having the 2D antennastructure, and “a” is a value representing phase difference between thefirst domain antenna group and the second domain antenna group.
 11. Amethod for receiving channel state information (CSI) from a base stationof a wireless communication system, the method comprising: providing auser equipment with channel state information-reference signal (CSI-RS)configuration information and transmitting a CSI-RS to the userequipment based on the CSI-RS configuration information; and receivingthe CSI, which is generated by the user equipment using the CSI-RS, fromthe user equipment, wherein the CSI includes precoding informationselected from a predetermined codebook, elements of the predeterminedcodebook are configured based on a precoding vector W, the precodingvector W is ${W = \begin{bmatrix}{W\; 1} \\{{aW}\; 2}\end{bmatrix}},$ W1 is a precoding vector applied to a first domainantenna group having a 2D antenna structure, W2 is a precoding vectorapplied to a second domain antenna group having the 2D antennastructure, and “a” is a value representing phase difference between thefirst domain antenna group and the second domain antenna group.
 12. Abase station for receiving channel state information (CSI) in a wirelesscommunication system, the base station comprising: a receiver; atransmitter; and a processor, wherein the processor is configured to:provide a user equipment with channel state information-reference signal(CSI-RS) configuration information, and control the transmitter totransmit a CSI-RS to the user equipment based on the CSI-RSconfiguration information, and control the receiver to receive the CSI,which is generated by the user equipment using the CSI-RS, from the userequipment, the CSI includes precoding information selected from apredetermined codebook, elements of the predetermined codebook areconfigured based on a precoding vector W, the precoding vector W is${W = \begin{bmatrix}{W\; 1} \\{{aW}\; 2}\end{bmatrix}},$ W1 is a precoding vector applied to a first domainantenna group having a 2D antenna structure, W2 is a precoding vectorapplied to a second domain antenna group having the 2D antennastructure, and “a” is a value representing phase difference between thefirst domain antenna group and the second domain antenna group.